qemurunner: fix undefined variable

While hacking on this I got an Exception. It's better to define
variables also in python.

Signe:-off-by: Adrian Freihofer <adrian.freihofer@siemens.com>
(From OE-Core rev: 1ea225a86cdee4ed932ede509d3351d5aecae497)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/utils/qemurunner.py

@@ -60,6 +60,7 @@ class QemuRunner:
         self.runqemutime = 120
         self.qemu_pidfile = 'pidfile_'+str(os.getpid())
         self.host_dumper = HostDumper(dump_host_cmds, dump_dir)
+        self.monitorpipe = None
 
         self.logger = logger