VSOCK: Introduce virtio-vsock-common.ko

This module contains the common code and header files for the following
virtio-vsock and virtio-vhost kernel modules.

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