Before HEAD^, we needed this because bdrv_co_flush() by itself would
only flush bs->file.  With HEAD^, bdrv_co_flush() will flush all
children on which a WRITE or WRITE_UNCHANGED permission has been taken.
Thus, vmdk no longer needs to do it itself.

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