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
libvirt: introduce new helper for getting libvirt domain

Introduce a get_domain() method to obtain a libvirt domain
from a Nova instance object. This slightly simplifies the
callers, avoiding the need for them to know that the lookup
is done based on the name (as opposed to ID or UUID).

Blueprint: libvirt-driver-class-refactor
Change-Id: Iea29950f3991a8051f48263f43c7b9b861d9e640
Merge "libvirt: remove pointless _get_host_uuid method"
Merge "libvirt: introduce new helper for getting libvirt domain"