Add printing support for vsockmon devices.

Add printing support for vsockmon devices.

Print Linux 4.12 vsockmon captures:

  # modprobe vsockmon
  # ip link add type vsockmon
  # ip link set vsockmon0 up
  # tcpdump -i vsockmon0
  16:25:24.987917 VIRTIO 3.1025 > 2.1234 CONNECT, length 76
  16:25:24.987963 VIRTIO 2.1234 > 3.1025 CONNECT, length 76
  16:25:26.568271 VIRTIO 3.1025 > 2.1234 PAYLOAD, length 82
  16:25:26.568512 VIRTIO 2.1234 > 3.1025 CONTROL, length 76
  16:25:28.411335 VIRTIO 3.1025 > 2.1234 DISCONNECT, length 76
  16:25:28.411628 VIRTIO 2.1234 > 3.1025 DISCONNECT, length 76

For more information about vsock see:
http://wiki.qemu.org/Features/VirtioVsock