libvirt: add '_' prefix to all get_guest_*_config methods

The get_guest_*_config 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: I76108c4992144cc7c3273bfd753aa76f7a821a56
Merge "libvirt: add '_' prefix to all get_guest_*_config methods"