libvirt: add '_' prefix to some get_host_* methods

The get_host_uuid, get_host_capabilities and get_host_cpu_for_guest
methods are all internal helpers so should have a '_' prefix to
indicate they are not part of public virt driver API.

Related-bug: #1333219
Change-Id: Ice22f52ef1f4ec0b950b5fdf303499ea5c2ba7be
Merge "libvirt: add '_' prefix to some get_host_* methods"