Lots of random fixes:

Rework the MMCallbackInfo callback invocation.
Always use g_error_literal() where it makes sense.
Replace sleep() calls, with timeouts to not block the whole MM.

src/mm-modem.c

@@ -20,8 +20,8 @@ async_op_not_supported (MMModem *self,
     MMCallbackInfo *info;
 
     info = mm_callback_info_new (self, callback, user_data);
-    info->error = g_error_new (MM_MODEM_ERROR, MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED,
-                               "%s", "Operation not supported");
+    info->error = g_error_new_literal (MM_MODEM_ERROR, MM_MODEM_ERROR_OPERATION_NOT_SUPPORTED,
+                                       "Operation not supported");
     mm_callback_info_schedule (info);
 }