libvirt: remove pointless _get_host_uuid method

The _get_host_uuid method only had one caller these days, so it
is a pretty pointless helper API.

Blueprint: libvirt-driver-class-refactor
Change-Id: Ibe0e5836a94295378f2208b220e452b61545c1c9
Merge "libvirt: remove pointless _get_host_uuid method"