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>