libvirt: remove volume_driver_method API

Replace the 'volume_driver_method' API with _connect_volume
and _disconnect_volume methods to remove the pointless
introspection based calling technique. This adds a '_' to
the names to show that they are internal methods only.

Related-bug: #1333219
Change-Id: Ife3fb60dd8aef8adf6bbdd2465971e156430fa0d
Merge "libvirt: remove volume_driver_method API"