qemu: cleanup code to release VNC websocket port

qemu: cleanup code to release VNC websocket port

VNC websocket port cleanup looks a bit repetetive. Let's set websocketReserved
flag whenever we reserve port (auto or not).

Also websocketReserved 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.

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