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>