Fix asyncio error when closing the app

Fix #310, https://github.com/GNS3/gns3-gui/issues/705

gns3server/modules/base_vm.py

@@ -61,7 +61,6 @@ class BaseVM:
 
     def __del__(self):
 
-        self.close()
         if self._temporary_directory is not None:
             if os.path.exists(self._temporary_directory):
                 shutil.rmtree(self._temporary_directory, ignore_errors=True)

gns3server/modules/qemu/qemu_vm.py

@@ -588,7 +588,7 @@ 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()
 
@@ -603,7 +603,7 @@ class QemuVM(BaseVM):
             log.info('Stopping QEMU VM "{}" PID={}'.format(self._name, self._process.pid))
             try:
                 self._process.terminate()
-                self._process.wait()
+                yield from self._process.wait()
             except subprocess.TimeoutExpired:
                 try:
                     self._process.kill()