acpi: Fix PCI hole handling on build_srat()

acpi: Fix PCI hole handling on build_srat()

The original SeaBIOS code used the RamSize variable, that was used by
SeaBIOS for the size of RAM below 4GB, not for all RAM. When copied to
QEMU, the code was changed to use the full RAM size, and this broke the
build_srat() code that handles the PCI hole.

Change build_srat() to use ram_size_below_4g instead of ram_size, to
restore the original behavior from SeaBIOS.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>