Instead of listing only monitor-owned BlockBackends in query-block, also
add those anonymous BlockBackends that are owned by a qdev device and as
such under the control of the user.

This allows using query-block to inspect BlockBackends for the modern
configuration syntax with -blockdev and -device.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>