I use Win7 VM's from modern.ie on both a Linux machine (running Archlinux 4.2.2 kernel) and a Win10 machine. The Win7 VM's are very convenient because they give you an evaluation period of 90 days and the trial can be rearmed up to 3 times for a total of 360 days of use. This is irrespective of whether you have saved a snapshot of your VM; once the snapshot is restored, the number of days that have elapsed since the first installation will be used to calculate how many evaluation days you have remaining.
I installed a Win7 VM for a Linux host just 30 days ago, but when I resumed from a save state, the VM told me my evaluation had expired! I thus rearmed the evaluation period using slmgr/rearm from the windows command prompt cmd.exe (which must be run as Administrator). I expected to see that I would be given 90 additional days of use, but I got the following:
From the screenshot (click above to expand) you can see that after the rearm, only 10 days are remaining (although 90 should be granted) and that there are 4 rearms remaining...
On a laptop running Windows 10 (host), the Win7 VM (guest) from modern.ie runs just fine in Virtualbox and the evaluation period counts down correctly. You can see that I have 54 days remaining on this VM:
I wonder what happened to the Win7 VM on the Linux host that caused the evaluation period timer to get messed up...
Postscript Dec. 20, 2015
The first time you boot your new Win7 VM, you will be asked whether you would like to go online to validate your installation (connect to the licensing server). If you don't connect to the licensing server at least once, your VM will only be valid for 10 days. Although it is possible to reset the counter 3 times, unless you validate online you will only be given 10 days of usage at most. I think what happened is that I accidentally closed the validation window the first time I booted the VM, so I was never given 90 days of full usage before a usage counter reset with slmgr/rearm