Make separate calls to libvirt volume

This is a follow up patch to
https://review.openstack.org/#/c/116998/, where libvirt
volume's connect_volume method were split into get_config and
connect_volume. The following patch changes libvirt driver
to call the appropriate volume method, get_config or
connect_volume, instead of the old monolithic call to
connect_volume. connect_volume will no longer need to return
the libvirt xml after this switch over. This will improve
performance and eliminate possible race conditions.

Change-Id: I0d0d7ba98c53ad7509c28bb4866c488c0a049d2f
Closes-Bug: #1359617
Merge "Make separate calls to libvirt volume"