savevm: Delete existing snapshots in all images

savevm: Delete existing snapshots in all images

When creating a snapshot we can run into the situation that the first disk
doesn't have a snapshot, but the second one does have one with the same name as
the new snapshot.

In this case, qemu doesn't recognize that there is a snapshot to be
overwritten, so it starts to save the new snapshot and errors out later when it
tries to snapshot the second image. With this patch, snapshots on secondary
images are overwritten just like on the first image.

Reported-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>