KVM: nSVM: extract preparation of VMCB for nested run

Split out filling svm->vmcb.save and svm->vmcb.control before VMRUN.
Only the latter will be useful when restoring nested SVM state.

This patch introduces no semantic change, so the MMU setup is still
done in nested_prepare_vmcb_save.  The next patch will clean up things.

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