CIDs 1162783 & 1162784: out-of-bounds access & write

 * NUM_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.