machine, Add default_machine_opts to QEMUMachine.

With this new field, we can specified which accelerator use to run the
machine, if the accelerator is not already specified by either a
configuration file or the command line options.

Currently, the only use will be made in the xenfv machine.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/hw/boards.h b/hw/boards.h
index 6f0f0d7..716fd7b 100644
--- a/hw/boards.h
+++ b/hw/boards.h
@@ -27,6 +27,7 @@
         no_cdrom:1,
         no_sdcard:1;
     int is_default;
+    const char *default_machine_opts;
     GlobalProperty *compat_props;
     struct QEMUMachine *next;
 } QEMUMachine;