qmp: fix __accept() in qmp.py

In QEMUMonitorProtocol, commit e9d17b68 removed the __sockfile creation
from __negotiate_capabilities(), which breaks _accept().  This causes
failures in qemu-io python based tests (i.e. tests 030 and 040).

This patch creates the sockfile in __accept() as well.
Signed-off-by: NJeff Cody <jcody@redhat.com>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Acked-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

QMP/qmp.py

@@ -96,6 +96,7 @@ def accept(self):
         @raise QMPCapabilitiesError if fails to negotiate capabilities
         """
         self.__sock, _ = self.__sock.accept()
+        self.__sockfile = self.__sock.makefile()
         return self.__negotiate_capabilities()
 
     def cmd_obj(self, qmp_cmd):