gluster: add support for PREALLOC_MODE_FALLOC

Add missing support for "preallocation=falloc" to the Gluster block
driver. This change bases its logic on that of block/file-posix.c and
removed the gluster_supports_zerofill() and qemu_gluster_zerofill()
functions in favour of #ifdef checks in an easy to read
switch-statement.

Both glfs_zerofill() and glfs_fallocate() have been introduced with
GlusterFS 3.5.0 (pkg-config glusterfs-api = 6). A #define for the
availability of glfs_fallocate() has been added to ./configure.

Reported-by: Satheesaran Sundaramoorthi <sasundar@redhat.com>
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Message-id: 20170528063114.28691-1-ndevos@redhat.com
URL: https://bugzilla.redhat.com/1450759
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
