q35: implement SMRAM.D_LCK

q35: implement SMRAM.D_LCK

Once the SMRAM.D_LCK bit has been set by the guest several bits in SMRAM
and ESMRAMC become readonly until the next machine reset.  Implement
this by updating the wmask accordingly when the guest sets the lock bit.
As the lock it itself is locked down too we don't need to worry about
the guest clearing the lock bit.

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