Add write mask for the smi enable register, so we can disable write
access to certain bits.  Open all bits on reset.  Disable write access
to GBL_SMI_EN when SMI_LOCK (in ich9 lpc pci config space) is set.
Write access to SMI_LOCK itself is disabled too.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>