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

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: Corey Minyard <cminyard@mvista.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>