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