Asyncio qemu fix and raise error if coroutine not used

gns3server/modules/qemu/qemu_vm.py

@@ -870,7 +870,7 @@ class QemuVM(BaseVM):
                     log.error("Could not start QEMU {}: {}\n{}".format(self.qemu_path, e, stdout))
                     raise QemuError("Could not start QEMU {}: {}\n{}".format(self.qemu_path, e, stdout))
 
-                self._set_process_priority()
+                yield from self._set_process_priority()
                 if self._cpu_throttling:
                     self._set_cpu_throttling()