virsh shutdown can be compared to pushing the power button on a server, that will normally tell the OS to shut down gracefully. Microsoft on the other hand looks on that as a security issue and have per default disabled the ability to shut down the server if no admin is logged in on the server. (Aparantly you cant do anything naughty if you have access to the server..)
This causes virsh shutdown to not function as intended for Windows servers.
Luckily, You can fix this by setting the following registry key to 1:
PS: If that does not help, check out the power settings i gpedit.msc, some information about that can be found @