Fix importing l2arc device by guid

After r292066, vdev_geom verifies both the vdev and pool guids of device
labels during open. However, spare and l2arc devices don't have pool guids,
so opening them by guid will fail (opening by path, when the pathname is
known, still succeeds). This change allows a vdev to be opened by guid if
the label contains no pool_guid, which is the case for inactive spares and
l2arc devices.

PR: 292066
Reported by: delphij
Reviewed by: delphij, smh
MFC after: 2 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D4861