KVM: selftests: Add "struct kvm_mmu" to track a given MMU instance

Add a "struct kvm_mmu" to track a given MMU instance, e.g. a VM's stage-1
MMU versus a VM's stage-2 MMU, so that x86 can share MMU functionality for
both stage-1 and stage-2 MMUs, without creating the potential for subtle
bugs, e.g. due to consuming on vm->pgtable_levels when operating a stage-2
MMU.

Encapsulate the existing de facto MMU in "struct kvm_vm", e.g instead of
burying the MMU details in "struct kvm_vm_arch", to avoid more #ifdefs in
____vm_create(), and in the hopes that other architectures can utilize the
formalized MMU structure if/when they too support stage-2 page tables.

No functional change intended.

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230230150.4150236-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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