qemu.py: Fix error message when qemu dies from signal

When qemu dies from a signal, the python code gets a negative
value for exitcode; but signal numbers are positive.  Copy the
pattern used in qemu-iotests/iotests.py for reporting a positive
value.

CC: qemu-trivial@nongnu.org
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20190111201330.14473-1-eblake@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

scripts/qemu.py

@@ -351,7 +351,7 @@ class QEMUMachine(object):
                 command = ' '.join(self._qemu_full_args)
             else:
                 command = ''
-            LOG.warn(msg, exitcode, command)
+            LOG.warn(msg, -exitcode, command)
 
         self._launched = False