fix vm shutdown when agent conf is enabled but is not running in the vm

Checking only vm configuration for choose the shutdown method causes it
to always fail, after reaching the timeout, if the qemu agent option in
the vm configuration is enabled but the agent is not installed and
active in the guest.
As I seen in the windows vm the agent also crashes in some cases, so
shutdown don't fail only if qemu guest agent is not installed or not
started.

Added check that agent is active when choosing agent shutdown method to
avoid certain shutdown failure in those cases.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
[FE: do not set flag to suppress warning when agent is not running]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>

Author: Fabio Fantoni via pve-devel

PVE/QemuServer.pm

@@ -6349,7 +6349,7 @@ sub _do_vm_stop {
 
     eval {
 	if ($shutdown) {
-	    if (defined($conf) && get_qga_key($conf, 'enabled')) {
+	    if (defined($conf) && get_qga_key($conf, 'enabled') && qga_check_running($vmid)) {
 		mon_cmd($vmid, "guest-shutdown", timeout => $timeout);
 	    } else {
 		mon_cmd($vmid, "system_powerdown");