acpi: fix tables for no-hpet configuration

acpi build tried to add offset of hpet table to rsdt even when hpet was
disabled.  If no tables follow hpet, this could lead to a malformed
rsdt.

Fix it up.

To avoid such errors in the future, rearrange code slightly to make it
clear that acpi_add_table stores the offset of the following table - not
of the previous one.

Reported-by: TeLeMan <geleman@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: qemu-stable@nongnu.org