python: Treat None-return of greeting cmd

In case qemu process dies the "monitor.cmd" returns None which gets
passed to the "__negotiate_capabilities" and leads to unhandled
exception. Let's only check the resp in case it has a value.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NWainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20200120071202.30646-1-ldoktor@redhat.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>

python/qemu/qmp.py

@@ -80,7 +80,7 @@ class QEMUMonitorProtocol:
             raise QMPConnectError
         # Greeting seems ok, negotiate capabilities
         resp = self.cmd('qmp_capabilities')
-        if "return" in resp:
+        if resp and "return" in resp:
             return greeting
         raise QMPCapabilitiesError