block/qapi: Add qdev device name to query-block

With -blockdev/-device, users can indirectly create anonymous
BlockBackends, while the state of such backends is still of interest. As
a preparation for making such BBs visible in query-block, make sure that
they can be identified even without a name by adding the ID/QOM path of
their qdev device to BlockInfo.

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