glusterfs : create volume through gluster block driver instead fuse mount point

forum user report slow qcow2 volume create with preallocated metadatas
http://forum.proxmox.com/threads/17471-GlusterFS-amp-Proxmox-Future-amp-QCOW2-Issues

(note that I can't reproduce it with qemu 1.7)

But redhat bugzilla have an entry about possible problem with volume is create through mount point.
https://bugzilla.redhat.com/show_bug.cgi?id=895830

So,It's better to manage it through gluster block driver directly.

(We need only the mount point to create directory and list images files)

Signed-off-by: Alexandre Derumier <aderumier@odiso.com>
bump version to 3.0-19