KVM: x86: unify callbacks to load paging root

Similar to what kvm-intel.ko is doing, provide a single callback that
merges svm_set_cr3, set_tdp_cr3 and nested_svm_set_tdp_cr3.

This lets us unify the set_cr3 and set_tdp_cr3 entries in kvm_x86_ops.
I'm doing that in this same patch because splitting it adds quite a bit
of churn due to the need for forward declarations.  For the same reason
the assignment to vcpu->arch.mmu->set_cr3 is moved to kvm_init_shadow_mmu
from init_kvm_softmmu and nested_svm_init_mmu_context.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -4932,7 +4932,6 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
 	context->update_pte = nonpaging_update_pte;
 	context->shadow_root_level = kvm_x86_ops->get_tdp_level(vcpu);
 	context->direct_map = true;
-	context->set_cr3 = kvm_x86_ops->set_tdp_cr3;
 	context->get_guest_pgd = get_cr3;
 	context->get_pdptr = kvm_pdptr_read;
 	context->inject_page_fault = kvm_inject_page_fault;
@@ -5079,7 +5078,6 @@ static void init_kvm_softmmu(struct kvm_vcpu *vcpu)
 	struct kvm_mmu *context = vcpu->arch.mmu;
 
 	kvm_init_shadow_mmu(vcpu);
-	context->set_cr3           = kvm_x86_ops->set_cr3;
 	context->get_guest_pgd     = get_cr3;
 	context->get_pdptr         = kvm_pdptr_read;
 	context->inject_page_fault = kvm_inject_page_fault;