Add the PCI version of vhost-user-vsock

Launch QEMU like this:

  qemu -chardev socket,path=/tmp/vm.vsock,id=chr0 \
       -device vhost-user-vsock-pci,chardev=chr0
Signed-off-by: NStefano Garzarella <sgarzare@redhat.com>
Message-Id: <20200522122512.87413-4-sgarzare@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>