Support for GLUSTERFS_DOMAIN

This patch introduces a new storage domain of type
GLUSTERFS_DOMAIN, which uses gluster as the storage backend.

In GLUSTERFS_DOMAIN, vdsm creates the storage domain by mounting
the gluster volume (akin to nfs mounting export path). VMs
created using this domain exploit the QEMU's gluster block
backend. Instead of accessing the vmdisk as a file path, it
accesses the vmdisk as a network disk device, served by gluster
server/volume.

This patch attempts to re-use nfsSD core logic (to an extent)
to support domain of type GLUSTERFS_DOMAIN.

Change-Id: I9ac37da88625f20d148beaf53bb6371c15b33ad7
Signed-off-by: Deepak C Shetty <deepakcs@linux.vnet.ibm.com>
Reviewed-on: http://gerrit.ovirt.org/6856
Reviewed-by: Dan Kenigsberg <danken@redhat.com>