block/qdev: Let 'drive' property fall back to node name

If a qdev block device is created with an anonymous BlockBackend (i.e.
a node name rather than a BB name was given for the drive property),
qdev used to return an empty string when the property was read. This
patch fixes it to return the node name instead.

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