boards.h: Ignore migration for SMBus devices on older machines

Migration capability is being added for pm_smbus and SMBus devices.
This change will allow backwards compatibility to be kept when
migrating back to an old qemu version.  Add a bool to the machine
class tho keep smbus migration from happening.  Future changes
will use this.
Signed-off-by: NCorey Minyard <cminyard@mvista.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Reviewed-by: NDr. David Alan Gilbert <dgilbert@redhat.com>

hw/i386/pc_piix.c

@@ -444,6 +444,7 @@ static void pc_i440fx_3_1_machine_options(MachineClass *m)
 
     pc_i440fx_4_0_machine_options(m);
     m->is_default = 0;
+    m->smbus_no_migration_support = true;
     m->alias = NULL;
     pcmc->pvh_enabled = false;
     compat_props_add(m->compat_props, hw_compat_3_1, hw_compat_3_1_len);

hw/i386/pc_q35.c

@@ -380,6 +380,7 @@ static void pc_q35_3_1_machine_options(MachineClass *m)
 
     pc_q35_4_0_machine_options(m);
     m->default_kernel_irqchip_split = false;
+    m->smbus_no_migration_support = true;
     m->alias = NULL;
     pcmc->pvh_enabled = false;
     compat_props_add(m->compat_props, hw_compat_3_1, hw_compat_3_1_len);

include/hw/boards.h

@@ -206,6 +206,7 @@ struct MachineClass {
     void (*numa_auto_assign_ram)(MachineClass *mc, NodeInfo *nodes,
                                  int nb_nodes, ram_addr_t size);
     bool ignore_boot_device_suffixes;
+    bool smbus_no_migration_support;
 
     HotplugHandler *(*get_hotplug_handler)(MachineState *machine,
                                            DeviceState *dev);