Add AF_VSOCK (virtio-vsock) support as an alternative to virtio-serial.

  $ qemu-system-x86_64 -device vhost-vsock-pci,guest-cid=3 ...
  (guest)# qemu-ga -m vsock-listen -p 3:1234
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NMichael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>