qemu-iotests: add unix socket help program

This program can do a sendmsg call to transfer fd with unix
socket, which is not supported in python2.

The built binary will not be deleted in clean, but it is a
existing issue in ./tests, which should be solved in another
patch.

Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>