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
libvirt: change representation of guest features

While the current acpi, apic & pae features are mere single elements,
further feature will require complex data structures. The libvirt
config class must thus change from using a simple string to represent
a feature, to a full class

Related-bug: #1400315
Change-Id: Ia6fa2602dd7a9e7623abcfd46c8b57d3631372ef
Merge "libvirt: add support for hyperv timer source with windows guests"
Merge "libvirt: change representation of guest features"