qemu: cleanup code to release VNC port

qemu: cleanup code to release VNC port

Code to release VNC port looks repetitive. The reason is there were
originally 2 functions to release ports - for auto and non-auto cases.

Also portReserved flag is not cleared on stop in case of reconnect with
auto port (flags is set on reconnect in qemuProcessGraphicsReservePorts call).
Yeah config is freed in the end of stopping domain but still.

Let's use this flag whenever we reserve port (auto or not). This makes
things clearer.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@openvz.org>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>