block/qapi: Add 'qdev' field to query-blockstats result

Like for query-block, the client needs to identify which BlockBackend
the returned data is for. Anonymous BlockBackends are identified by the
device model they are attached to. Add a 'qdev' field that contains the
qdev ID or QOM path of the attached device model.

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