virtfs-proxy-helper: add missing long option terminator The getopt_long(3) long options array must have a zeroed terminator. This patch solves a segmentation fault when an unknown command-line option is encountered: $ fsdev/virtfs-proxy-helper --help Segmentation fault (core dumped) Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>