iommu: KVM: Split "struct vcpu_data" into separate AMD vs. Intel structs

iommu: KVM: Split "struct vcpu_data" into separate AMD vs. Intel structs

Split the vcpu_data structure that serves as a handoff from KVM to IOMMU
drivers into vendor specific structures.  Overloading a single structure
makes the code hard to read and maintain, is *very* misleading as it
suggests that mixing vendors is actually supported, and bastardizing
Intel's posted interrupt descriptor address when AMD's IOMMU already has
its own structure is quite unnecessary.

Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Link: https://lore.kernel.org/r/20250611224604.313496-33-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>