char: Fix closing of various char devices (Jan Kiszka)

This patch fixes several issues around closing char devices. Affected
were pty (timer was left behind, even running), udp (no close handling
at all) and tcp (missing async IO handler cleanup). The bugs either
caused segfaults or stalled the qemu process. So far, hot-unplugging USB
serial adapters suffered from this.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6911 c046a42c-6fe2-441c-8c8c-71466251a162