libvirt: Refresh volume connection_info after volume snapshot

The following patch is related to the guest assisted snapshot
functionality.  When you take a snapshot of a volume
(e.g. GlusterFS) attached to a running instance, a new snapshot
file is created, i.e. volume-<uuid>.<snapshot-uuid>.  The
instance uses this file as the active volume.  If you shutdown
and restart the instance, nova will reattach the base volume
(volume-<uuid>) to the instance instead of the snapshot volume
(volume-<uuid>.<snapshot-uuid>).  The expected behavior is to
have the snapshot volume reattach to the instance.  This is
caused by stale data being returned from the database when
_get_instance_volume_block_device_info is called during
_power_on.  To fix this bug, this patch calls
refresh_connection_info to update the database in both
volume_snapshot_create and _volume_snapshot_delete methods of the
libvirt driver.

Change-Id: I0f340a3f879580e7981d97863bc299e33d71aa84
Closes-Bug: #1304695
Merge "libvirt: Refresh volume connection_info after volume snapshot"