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>

Author: sean-jc

tools/testing/selftests/kvm/include/kvm_util.h

@@ -116,7 +116,12 @@ struct kvm_vm {
 	uint32_t dirty_ring_size;
 	uint64_t gpa_tag_mask;
 
+	/*
+	 * "mmu" is the guest's stage-1, with a short name because the vast
+	 * majority of tests only care about the stage-1 MMU.
+	 */
 	struct kvm_mmu mmu;
+	struct kvm_mmu stage2_mmu;
 
 	struct kvm_vm_arch arch;