I am a developer and often need to have a clean machine to test with.  I use VirtualBox for virtualization on Windows 7 Ultimate x64.

My MSDN subscription gives me five product keys for Windows 7 Ultimate x64.  I created a base Virtual Machine using one of the keys and applied all Windows Update.  Call that machine W7Base.

When I need a VM for testing I clone the W7Base VDI file (equivalent to a VHD) and create a new Virtual Machine.  I then boot the machine and change the product key to one of the remaining MSDN subscription key.  I can then apply whatever soffware/modifications needed to get it to be a test system.

The MSDN subscription provides a single mutiple activation Windows 10 Pro key.

Now the Windows 10 activation process creates a unique hash of the machine fingerprint and that and the product key are used for initial activation.  After that you can rebuild the machine without entering a product key as the hashed fingerprint is enough to activate.

My question is what happens when I try and use the cloned machine.  The hashed fingerprint will be different.  Will I get a prompt for a product key and then be able to activate it using the multi activation product key or the multi activation product key be associated with the fingerprint of the W7Base machine?

The physical hardware ID and the virtual one will obviously be different.  I have not tested the limits of the "multi" key activation yet but know I have been prompted and activated.

What I have done is to use one of the 5 Pro MSDN key to activate a virtual machine ID.  I have been able to scrub and re-install VMS as necessary in Hyper-v.  I suspect VB would be the same

I don't think you'll have a problem...  here is why

I have copied an entire VirtualBox machine folder (the *.vbox, *.vdi, snapshots, logs, etc) to a removable drive.   That way I can run my virtual PC from practically anywhere. However, the removable drive is slow, so occasionally I have copied the entire folder to the "visited" PCs drive instead.

I have also performed a "full clone" operation from VBoxManage on my main PC.

Not once did any of this invalidate my Activation of the guest OS... I suspect because the underlying hardware has not changed (since it has been virtualized).