libvirt: add '_' prefix to host state information methods

The methods used by the HostState class 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: Ia9b2e966071d5a842193421394aa0850beb3fdba
Merge "libvirt: add '_' prefix to host state information methods"