iotests: Flush in iotests.py's QemuIoInteractive

After issuing a command, flush the pipe.  This does not change anything
in Python 2, but it makes a difference in Python 3.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20181022135307.14398-3-mreitz@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

tests/qemu-iotests/iotests.py

@@ -178,6 +178,7 @@ class QemuIoInteractive:
         cmd = cmd.strip()
         assert cmd != 'q' and cmd != 'quit'
         self._p.stdin.write(cmd + '\n')
+        self._p.stdin.flush()
         return self._read_output()