AMD-vi: Fortify IVHD device_identify process

• Use malloc(9) to allocate ivhd_hdrs list. The previous assumption that there are at most 10 IVHDs in a system is not true. A counter example would be a system with 4 IOMMUs, and each IOMMU is related to IVHDs type 10h, 11h and 40h in the ACPI IVRS table.
• Always scan through the whole ivhd_hdrs list to find IVHDs that has the same DeviceId but less prioritized IVHD type.

Sponsored by: The FreeBSD Foundation
MFC with: 74ada297e897
Reviewed by: grehan
Approved by: lwhsu (mentor)
Differential Revision: https://reviews.freebsd.org/D29525