common: add --qemu option to override configs

run

@@ -127,7 +127,7 @@ def main(args, extra_args=None):
     if common.image is None:
         raise Exception('Baremetal ELF file not found. Tried:\n' + '\n'.join(paths))
     cmd = debug_vm.copy()
-    if args.gem5:
+    if common.emulator == 'gem5':
         if args.baremetal is None:
             if not os.path.exists(common.rootfs_raw_file):
                 if not os.path.exists(common.qcow2_file):
@@ -359,7 +359,7 @@ def main(args, extra_args=None):
             if args.baremetal is None:
                 cmd.extend(append)
     if args.tmux is not None:
-        if args.gem5:
+        if common.emulator == 'gem5':
             subprocess.Popen([os.path.join(common.root_dir, 'tmu'),
                 'sleep 2;./gem5-shell -n {} {}' \
                 .format(args.run_id, args.tmux)
@@ -386,7 +386,7 @@ def main(args, extra_args=None):
         out_file = common.termout_file
     common.run_cmd(cmd, cmd_file=common.run_cmd_file, out_file=out_file, extra_env=extra_env)
     # Check if guest panicked.
-    if args.gem5:
+    if common.emulator == 'gem5':
         # We have to do some parsing here because gem5 exits with status 0 even when panic happens.
         # Grepping for '^panic: ' does not work because some errors don't show that message.
         panic_msg = b'--- BEGIN LIBC BACKTRACE ---$'