qemu: hotplug: Add debug log when dispatching device removal to existing thread

Note that the waiting thread is signaled in the debug logs to simplify
debugging.

src/qemu/qemu_hotplug.c

@@ -4383,6 +4383,7 @@ qemuDomainSignalDeviceRemoval(virDomainObjPtr vm,
     qemuDomainObjPrivatePtr priv = vm->privateData;
 
     if (STREQ_NULLABLE(priv->unplug.alias, devAlias)) {
+        VIR_DEBUG("Removal of device '%s' continues in waiting thread", devAlias);
         qemuDomainResetDeviceRemoval(vm);
         priv->unplug.status = status;
         virDomainObjBroadcast(vm);