KVM: selftests: Add a stage-2 MMU instance to kvm_vm

KVM: selftests: Add a stage-2 MMU instance to kvm_vm

Add a stage-2 MMU instance so that architectures that support nested
virtualization (more specifically, nested stage-2 page tables) can create
and track stage-2 page tables for running L2 guests.  Plumb the structure
into common code to avoid cyclical dependencies, and to provide some line
of sight to having common APIs for creating stage-2 mappings.

As a bonus, putting the member in common code justifies using stage2_mmu
instead of tdp_mmu for x86.

Reviewed-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Link: https://patch.msgid.link/20251230230150.4150236-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>