qemu.py: use poll() instead of 'returncode'

The 'returncode' Popen attribute is not guaranteed to be updated. It
actually depends on a call to either poll(), wait() or communicate().

On the other hand, poll() will: "Check if child process has terminated.
Set and return returncode attribute."

Let's use the poll() to check whether the process is running and to get
the updated process exit code, when the process is finished.
Reviewed-by: NFam Zheng <famz@redhat.com>
eviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NAmador Pahim <apahim@redhat.com>
Message-Id: <20180122205033.24893-5-apahim@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

scripts/qemu.py

@@ -150,12 +150,12 @@ class QEMUMachine(object):
             raise
 
     def is_running(self):
-        return self._popen is not None and self._popen.returncode is None
+        return self._popen is not None and self._popen.poll() is None
 
     def exitcode(self):
         if self._popen is None:
             return None
-        return self._popen.returncode
+        return self._popen.poll()
 
     def get_pid(self):
         if not self.is_running():