hw/i386/acpi-build: Rely on machine->device_memory when building SRAT

hw/i386/acpi-build: Rely on machine->device_memory when building SRAT

We're already looking at machine->device_memory when calling
build_srat_memory(), so let's simply avoid going via
PC_MACHINE_DEVMEM_REGION_SIZE to get the size and rely on
machine->device_memory directly.

Once machine->device_memory is set, we know that the size > 0. The code now
looks much more similar the hw/arm/virt-acpi-build.c variant.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Eduardo Habkost <eduardo@habkost.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20230623124553.400585-8-david@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>