I've got Windows 10 installed in VirtualBox 5.0, upgraded to Build 10240 (which is supposed to be the Final, same as RTM release).
It started out as an earlier Insider Build, upgrading to newer builds periodically along the way.
Just for giggles, I downloaded the build 10240 .iso I found and did test installs of it in VMWare Player and KVM (using a Kubuntu 14.10 host OS). I had read that they had discontinued the keys for the earlier insider builds. So, rather than installing the .iso I installed originally, i went ahead and downloaded the latest .iso instead.
It installed that way (fresh installs), too. But, I could not activate it after a clean install (keys found did not work with it).
IOW, it should install fine in a Virtual Machine. But, unless you have an install that was upgraded from earlier Insider Previews, it probably won't activate for you (although you should be able to use it with most features for a while without any activation).
All of the test installs I have (under VirtualBox, KVM and VMWare Player) are using Kubuntu 14.10 as the host Operating System, running 64 Bit Win 10 Pro as the Guest OS. But, the only Win 10 build that I have that is activated is the one running in Virtual Box (5.0) that had been updated from earlier insider builds through the current build 10240.
BTW, just for giggles, I used the .vmdk (virtual drive) from the Win 10 install in VMWare Player with VirtualBox, too. Basically, I just created a new VM in VirtualBox, then instead of creating a new Virtual Drive, I pointed the Machine Storage settings to the .vmdk created when I installed Win 10 in VMWare Player and it worked fine that way, too.
But, you do need to create a SAS Controller in the VirtualBox machine when you attach a .vmdk from VMWare Player (even though VMWare Player considers it to be a SCSI drive). That's just a known compatibility problem, where a SCSI Controller type in VirtualBox won't work with a .vmdk created as SCSI by VMWare Player. You need to use a SAS Controller Type with VirtualBox instead.
Doing it that way, I can start the same Windows 10 installation (from the same .vmdk virtual drive) using the machine setup in VMWare Player, or the machine I setup in VirtualBox (since both are pointed at the same .vmdk file that is the virtual drive).
Anyway, to repeat, it should install and work fine in VirtualBox (at least 5.0), so i don't know why your install got stuck). You have both VirtualBox and the VirtualBox Extension Pack installed, right? I've got both installed in a Kubuntu 14.10 installation on my wife's laptop.
Note that I also had dkms installed. That's because Kubuntu has a bad habit of frequent Kernel Updates. So, as long as you have dkms installed, when you install VirtualBox and the VirtualBox Extension Pack, the kernel modules for it are added to dkms. That way, they get rebuilt automatically after each kernel update. If I use this command as root, I can see that (vboxhost modules being rebuilt with each linux kernel update):
Anyway, Win 10 will ask you to enter a product key more than once during the install (but, I ended up just telling it to skip, perform later, etc.) each time to get a complete installation when trying it with the latest .iso.
The VM that started out with an earlier insider build (using the Microsoft provided keys for it), that updated through 10240 (by downloading and doing the upgrades to later builds peridocally) is still activated and getting updates.
IOW, have mulitiple VMs setup in VirtualBox with Win 10 installs. One of them was originally created using an earlier insider build, and it's been updated through 10240. That's the install that is activated.
Another one was a machine using a clean install of the 10240 .iso It is not activated.
The third one is a machine that is pointed at a .vmdk file that was created using a 10240 .iso to install Win 10 in VMWare Player. It is not activated either.
I've got yet another Win 10 Virtual Machine installed in KVM (using a Win 10 Pro build 10240 .iso) and it is not activated either (but, it does install and run that way, just selecting Win 7 or higher as the OS type).
Basically, I was just testing the performance in different types of Virtual Machines that way (KVM versus VirtualBox versus VMWare Player) using my wife's laptop running Kubuntu 14.10 as the host. But, the only one that is activated is the VM in VirtualBox that started with an earlier insider build and was upgraded through 10240.
IOW, you probably won't be able to activate a new clean install now (unless you have a legit .iso and product keys for it that are not the "generic" keys). Instead, you'd have needed to start out with an earlier insider build that you activated, and let update itself until it finally upgraded to build 10240 (which should be the same as the final RTM build).