KVM: x86/mmu: Rename kvm_mmu->get_cr3() to ->get_guest_pgd()

Rename kvm_mmu->get_cr3() to call out that it is retrieving a guest
value, as opposed to kvm_mmu->set_cr3(), which sets a host value, and to
note that it will return something other than CR3 when nested EPT is in
use.  Hopefully the new name will also make it more obvious that L1's
nested_cr3 is returned in SVM's nested NPT case.

No functional change intended.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>