gluster: Fix blockdev-add with server.N.type=unix

gluster: Fix blockdev-add with server.N.type=unix

The legacy command line interface gets the socket path from an option
called 'socket'. QAPI in contract uses SocketAddress, where the
corresponding option is called 'path'.

Fix the gluster block driver to accept both 'socket' and 'path', with
'path' being the preferred syntax.

https://bugzilla.redhat.com/show_bug.cgi?id=1545155

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 20180403110810.25624-1-kwolf@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>