oVirt used a quirk in the pre-blockdev semantics of drive-mirror which
opened the backing chain of the mirror destination only once
'block-job-complete' was called.

Our introduction of blockdev made qemu open the backing chain images
right at the start of the job. This broke oVirt's usage of this API
because they copy the data into the backing chain during the time the
block copy job is running.

Re-introduce late open of the backing chain if qemu allows us to use
blockdev-snapshot on write-only nodes as it can be used to install the
backing chain even for an existing image now.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>