block: Allow specifying child options in reopen

If the child was defined in the same context (-drive argument or
blockdev-add QMP command) as its parent, a reopen of the parent should
work the same and allow changing options of the child.

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