Add option for QEMU Gluster libgfapi support

If gluster is present in qemu_allowed_storage_drivers, glusterfs's
backend will pass a disk configuration to QEMU. This allows QEMU to
access the volume using libgfapi rather than mounting GlusterFS via
fuse.

Implements blueprint glusterfs-native-support
DocImpact

Co-authored-by: Flavio Percoco <flavio@redhat.com>
Change-Id: I939cac05ee333b77abcaa9bd132d5eb3308e60ff
Merge "Add option for QEMU Gluster libgfapi support"