storage: document gluster pool

storage: document gluster pool

Add support for a new <pool type='gluster'>, similar to
RBD and Sheepdog.  Terminology wise, a gluster volume
forms a libvirt storage pool, within the gluster volume,
individual files are treated as libvirt storage volumes.

* docs/schemas/storagepool.rng (poolgluster): New pool type.
* docs/formatstorage.html.in: Document gluster.
* docs/storage.html.in: Likewise, and contrast it with netfs.
* tests/storagepoolxml2xmlin/pool-gluster.xml: New test.
* tests/storagepoolxml2xmlout/pool-gluster.xml: Likewise.
* tests/storagepoolxml2xmltest.c (mymain): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>