libvirt: add support for hyperv timer source with windows guests

The hyperv timer source enables Windows guests to keep accurate
time, much like kvmclock does for Linux guests. It should be
enabled for all windows guests, as it doesn't harm older windows
versions which don't know about it.

Related-bug: #1400315
Change-Id: I6af05b4e8e0b2145733c0f301c04555da23b7da8
Merge "libvirt: add support for hyperv timer source with windows guests"