If a qcow2 image specifies a backing file format that doesn't correspond
to any format driver that qemu knows, we shouldn't fall back to probing,
but simply error out.

Not looking up the backing file driver in bdrv_open_backing_file(), but
just filling in the "driver" option if it isn't there moves us closer to
the goal of having everything in QDict options and gets us the error
handling of bdrv_open(), which correctly refuses unknown drivers.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 1416935562-7760-4-git-send-email-kwolf@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>