VSOCK: extract utility functions from vsock_diag_test.c

Move useful functions into a separate file in preparation for more
vsock test programs.

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