python/aqmp: fix race condition in legacy.py

legacy.py provides a synchronous model. iotests frequently uses this
paradigm:

 - create QMP client object
 - start QEMU process
 - await connection from QEMU process

In the switch from sync to async QMP, the QMP client object stopped
calling bind() and listen() during the QMP object creation step, which
creates a race condition if the QEMU process dials in too quickly.

With refactoring out of the way, restore the former behavior of calling
bind() and listen() during __init__() to fix this race condition.

Signed-off-by: John Snow <jsnow@redhat.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20220225205948.3693480-10-jsnow@redhat.com
[Expanded commit message. --js]
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/aqmp/legacy.py

@@ -57,7 +57,7 @@ class QEMUMonitorProtocol(qemu.qmp.QEMUMonitorProtocol):
         self._timeout: Optional[float] = None
 
         if server:
-            self._aqmp._bind_hack(address)  # pylint: disable=protected-access
+            self._sync(self._aqmp.start_server(address))
 
     _T = TypeVar('_T')
 
@@ -90,10 +90,7 @@ class QEMUMonitorProtocol(qemu.qmp.QEMUMonitorProtocol):
         self._aqmp.await_greeting = True
         self._aqmp.negotiate = True
 
-        self._sync(
-            self._aqmp.start_server_and_accept(self._address),
-            timeout
-        )
+        self._sync(self._aqmp.accept(), timeout)
 
         ret = self._get_greeting()
         assert ret is not None