kvm: nVMX: Refactor nested_get_vmcs12_pages()

Perform the checks on vmcs12 state early, but defer the gpa->hpa lookups
until after prepare_vmcs02. Later, when we restore the checkpointed
state of a vCPU in guest mode, we will not be able to do the gpa->hpa
lookups when the restore is done.

Signed-off-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>