ich9: add smm_enabled field and arguments

Q35's ACPI device is hard-coding SMM availability to KVM.  Place the
logic where the board is created instead, so that it will be possible
to override it.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>