block: Move bdrv_attach_child() calls up the call chain

Let the callers of bdrv_open_inherit() call bdrv_attach_child(). It
needs to be called in all cases where bdrv_open_inherit() succeeds (i.e.
returns 0) and a child_role is given.

bdrv_attach_child() is moved upwards to avoid a forward declaration.

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