qemu.py: Add "wait()" method

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

scripts/qemu.py

@@ -215,6 +215,13 @@ class QEMUMachine(object):
                 LOG.debug('Output: %r', self._iolog)
             raise
 
+    def wait(self):
+        '''Wait for the VM to power off'''
+        self._popen.wait()
+        self._qmp.close()
+        self._load_io_log()
+        self._post_shutdown()
+
     def shutdown(self):
         '''Terminate the VM and clean up'''
         if self.is_running():