VSOCK: add tools/testing/vsock/vsock_diag_test

This patch adds tests for the vsock_diag.ko module.

These tests are not self-tests because they require manual set up of a
KVM or VMware guest.  Please see tools/testing/vsock/README for
instructions.

The control.h and timeout.h infrastructure can be used for additional
AF_VSOCK tests in the future.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>