gluster: Prepare for SocketAddressFlat extension

gluster: Prepare for SocketAddressFlat extension

qemu_gluster_glfs_init() and qemu_gluster_parse_json() rely on the
fact that SocketAddressFlatType has only two members
SOCKET_ADDRESS_FLAT_TYPE_INET and SOCKET_ADDRESS_FLAT_TYPE_UNIX.
Correct, but won't stay correct.  Make them more robust.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Jeff Cody <[email protected]>
Message-id: 1490895797[email protected]
Signed-off-by: Max Reitz <[email protected]>