CIDs 1162783 & 1162784: out-of-bounds access & write in MSI.

* The number of IO vectors is not 256 on x86, but rather 224 as set by
  NUM_IO_VECTORS in "arch_int.h".
* Jessicah mentioned hearing about MSI crashes before, but that was a
  few weeks ago.
* These were the only CIDs in the MSI code.

Signed-off-by: Michael Lotz <mmlr@mlotz.ch>