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: introduce new helper for getting libvirt domain"