LVM migration: Check if name is equal to dest_vg

The existence check of the destination volume group is wrong. It
currently checks if there's a VG with a name equal to the source's vg
name instead of checking if it's equal to the dest_vg.

Closes-bug: #1258203
Change-Id: Ia5d4acb24b94c6aa832107c7eb4b6996985af97f
Add RBD test for volume not existing during delete

For cases when the volume does not exist in the backend, but Cinder
still knows about it, we just return from the rbd driver and let Cinder
do its thing.

Change-Id: I6150756bdd48259ec8d550fa7a5dc7a9f882ca22
Merge "LVM migration: Check if name is equal to dest_vg"
Merge "Add RBD test for volume not existing during delete"