runqemu: don't fail during check_arg_machine()

If DEPLOY_DIR_IMAGE doesn't exist during check_arg_machine() we
will attempt to guess a suitable value later when check_and_set()
calls validate_paths(), therefore this shouldn't raise an exception

(From OE-Core rev: ed8d6f391c567048bd50dc3234804915f8212cef)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/runqemu

@@ -332,7 +332,7 @@ class BaseConfig(object):
             self.set_machine_deploy_dir(arg, deploy_dir_image)
         else:
             logger.error("%s not a directory valid DEPLOY_DIR_IMAGE" % deploy_dir_image)
-            raise Exception("Failed to set MACHINE to %s. Unknown arg: %s" % (arg, arg))
+            self.set("MACHINE", arg)
 
     def check_args(self):
         unknown_arg = ""