Add support for gluster backing store

Gluster is a distributed file system (www.gluster.org). The
backing store gives block access protocols such as iSCSI
access to data within gluster.

Gluster data resides on volumes which are physically located
on servers. A "LUN" is represented by a file within
a volume. To specify which file to use:

--bstype=glfs
--backing-store="volume@hostname:filename"

Optionally, logs may be specified. See README.glfs

All I/Os are synchronous.  This is based off the rdrw and
rbd backend storage drivers.

Signed-off-by: Dan Lambright <dlambrig@redhat.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>