Refactor libvirt volume driver classes to reduce duplication

Most of the libvirt volume driver class repeat the same setup
of the vconfig.LibvirtConfigGuestDisk() instance. Create a
more suitable base class to hold this common setup and simplify
all the other subclasses accordingly

Change-Id: I9a5a70ccf88a1e55cdb120c8c6d29e6847774691
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Refactor libvirt volume driver classes to reduce duplication"
Merge remote-tracking branch 'citrix/master' into build-temp