virtio/vsock: defines and constants for SEQPACKET

Add set of defines and constants for SOCK_SEQPACKET support
in vsock.

Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>