qtest driver always uses signals to kill qemu
no need to report it, whatever the accelerator state.

Add API to detect qtest driver, and suppress reporting
signals in this case.
Reported-by: NAndreas Färber <afaerber@suse.de>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NAndreas Färber <afaerber@suse.de>