virtio/vsock: add 'VIRTIO_VSOCK_SEQ_EOR' bit.

This bit is used to handle POSIX MSG_EOR flag passed from
userspace in 'send*()' system calls. It marks end of each
record and is visible to receiver using 'recvmsg()' system
call.

Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210903123225.3273425-1-arseny.krasnov@kaspersky.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>