libvirt: remove_logical_volumes should remove each separately

Currently, remove_logical_volumes() is trying to remove
all of the provided volumes path in a single lvremove command,
while retrying it several times.
This is causing the command to fail, when the deleted volumes
are reappearing in the next iteration of the retry loop.

Handling the removal of each volume separately
and raising an exception for volumes which failed to be removed.
This will increase the chance for individual volume to be removed.

Closes-Bug: #1282423
Change-Id: Ib25f5603929c3038716661a589a41116b4c48909
ec2: Convert instance_get_by_uuid calls to objects

Convert a couple uses of db.instance_get_by_uuid() to use the Instance
object instead.

Change-Id: I6c317c72661643d46ea0577d6efe87a24bf98b74
Merge "libvirt: remove_logical_volumes should remove each separately"
Merge "ec2: Convert instance_get_by_uuid calls to objects"