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: change representation of guest features"