block: Use blk_{commit,flush}_all() consistently

Replace bdrv_commmit_all() and bdrv_flush_all() by their BlockBackend
equivalents.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>