- docs/formatstorage.html.in: document 'zfs' pool type, add it
   to a list of pool types that could use source physical devices
 - docs/storage.html.in: update a ZFS pool example XML with
   source physical devices, mention that starting from 1.2.9 a
   pool could be created from this devices by libvirt and in earlier
   versions user still has to create a pool manually
 - docs/drvbhyve.html.in: add an example with ZFS pools