scripts/qemu.py: allow adding to the list of extra arguments

Tests will often need to add extra arguments to QEMU command
line arguments.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20180530184156.15634-3-crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

scripts/qemu.py

@@ -359,3 +359,9 @@ class QEMUMachine(object):
         of the qemu process.
         '''
         return self._iolog
+
+    def add_args(self, *args):
+        '''
+        Adds to the list of extra arguments to be given to the QEMU binary
+        '''
+        self._args.extend(args)