storage: implement rudimentary glusterfs pool refresh

storage: implement rudimentary glusterfs pool refresh

Actually put gfapi to use, by allowing the creation of a gluster
pool.  Right now, all volumes are treated as raw and directories
are skipped; further patches will allow peering into files to
allow for qcow2 files and backing chains, and reporting proper
volume allocation.  This implementation was tested against Fedora
19's glusterfs 3.4.1; it might be made simpler by requiring a
higher minimum, and/or require more hacks to work with a lower
minimum.

* src/storage/storage_backend_gluster.c
(virStorageBackendGlusterRefreshPool): Initial implementation.
(virStorageBackendGlusterOpen, virStorageBackendGlusterClose)
(virStorageBackendGlusterRefreshVol): New helper functions.

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