x86 iommu x86_unit_common: expand hw completion write-out area to 8 bytes

in preparation to share it with AMD IOMMU. AMD writes out 8 bytes. We
use 32-bit completion sequence numbers, and CPUs are little-endian. So
the expansion is acceptable on Intel.

Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week