KVM: selftests: Plumb "struct kvm_mmu" into x86's MMU APIs

KVM: selftests: Plumb "struct kvm_mmu" into x86's MMU APIs

In preparation for generalizing the x86 virt mapping APIs to work with
TDP (stage-2) page tables, plumb "struct kvm_mmu" into all of the helper
functions instead of operating on vm->mmu directly.

Opportunistically swap the order of the check in virt_get_pte() to first
assert that the parent is the PGD, and then check that the PTE is present,
as it makes more sense to check if the parent PTE is the PGD/root (i.e.
not a PTE) before checking that the PTE is PRESENT.

No functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
[sean: rebase on common kvm_mmu structure, rewrite changelog]
Link: https://patch.msgid.link/20251230230150.4150236-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>