confidential guest support: Move side effect out of machine_set_memory_encryption()

confidential guest support: Move side effect out of machine_set_memory_encryption()

When the "memory-encryption" property is set, we also disable KSM
merging for the guest, since it won't accomplish anything.

We want that, but doing it in the property set function itself is
thereoretically incorrect, in the unlikely event of some configuration
environment that set the property then cleared it again before
constructing the guest.

More importantly, it makes some other cleanups we want more difficult.
So, instead move this logic to machine_run_board_init() conditional on
the final value of the property.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>