qemu-nbd: simplify nbd_trip

Use TCP_CORK to remove a violation of encapsulation, that would later
require nbd_trip to know too much about an NBD reply.

We could also switch to sendmsg (qemu_co_sendv) later, it is even
easier once coroutines are in.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
