gluster: Get size information of a gluster volume.

New vdsm gluster verb to get free, used and total size of
gluster volume. This verb uses libgfapi to get the statistics
related to volume. This patch makes use of ctypes to utilize
the libgfapi. This patch also enhances glusterVolumeStatus
verb to return volume size info as well when status option
is set to detail.

verb: glusterVolumeStatsInfoGet

Output format:
{"sizeTotal": LONG as STR, "sizeFree": LONG as STR, "sizeUsed": LONG as STR}

Change-Id: If5d622738ae955eb4002f56fc73adb4f07f0b857
Signed-off-by: darshan n <dnarayan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/28581
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Bala.FA <barumuga@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>