-serial tcp should hang up when DTR goes low

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1213196

In keeping with the spirit of serial modem control signals, de-asserting
DTR should cause the TCP connection to break; asserting DTR should cause
QEMU to initiate a new connection or for it to accept another (in server
mode; this may involve waiting for one to arrive, too).