acpi: update RSDP on guest access

acpi: update RSDP on guest access

RSDT offset can change across reboots and that makes
immutable RSDP, which is build at startup, point to
incorrect place in ACPI table blob. That results in
BIOS corrupting tables and guest OS failing to find
ACPI tables.
We really should have put it in a ROM region, but
we can't change that for old machine types,
let's just set the callback and update it explicitly.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marcel Apfelbaum <marcel@redhat.com>