block: Unify bdrv_child_cb_attach()

block: Unify bdrv_child_cb_attach()

Make bdrv_child_cb_attach() call bdrv_backing_attach() for children with
a COW role (and drop the reverse call from bdrv_backing_attach()), so it
can be used for any child (with a proper role set).

Because so far no child has a proper role set, we need a temporary new
callback for child_backing.attach that ensures bdrv_backing_attach() is
called for all COW children that do not have their role set yet.

(Also, move bdrv_child_cb_attach() down to group it with
bdrv_inherited_options().)

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200513110544.176672-13-mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>