QED's opaque data includes a pointer back to the BlockDriverState.
This breaks when bdrv_append shuffles data between bs_new and bs_top.
To avoid this, add a "rebind" function that tells the driver about
the new relationship between the BlockDriverState and its opaque.

The patch also adds rebind to VVFAT for completeness, even though
it is not used with live snapshots.
Reviewed-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>