On some machines that support I/O hot-plugging,
it happens that after boottime one or more IO SAPICs appear
after hot-plug event.  Even in that case, ACPI _PRT entries
can exist for devices behind those IO SAPICs at boottime
for future use.

Currently iosapic.c will give up parsing _PRT entries
once one of them hits such a non-existent IO SAPIC.

This patch fixes the problem on 2.5 ia64 bk tree.
For 2.4, we don't have this problem now.