Verify that --pwd and --cwd are not both set

Signed-off-by: Andy Lutomirski <luto@kernel.org>
diff --git a/virtme/commands/run.py b/virtme/commands/run.py
index ce00154..78782ca 100644
--- a/virtme/commands/run.py
+++ b/virtme/commands/run.py
@@ -492,7 +492,9 @@
             return 1
         kernelargs.append('virtme_chdir=%s' % rel_pwd)
 
-    if args.cwd:
+    if args.cwd is not None:
+        if args.pwd:
+            arg_fail('--pwd and --cwd are mutually exclusive')
         rel_cwd = os.path.relpath(args.cwd, args.root)
         if rel_cwd.startswith('..'):
             print('specified working directory is not contained in the root')