kvm tools: Respect guest tcp window size

kvm tools: Respect guest tcp window size

Respect guest tcp window size and stop sending tcp segments to guest
if guest's receive window is closed.

This fixes the TCP hang I'm seeing where guest and host are transferring
big chuck of data.

This problem was not triggered when guest and external host
communicates, probably because guest to external host communication
walks through real network and is much slower than guest and host
communication. Thus, guest's receive window has little chance to be
closed.

v2: use pthread_cond_wait to wait

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>