KVM: Fix some out-dated function names in comment

Since commit b1346ab2 ("KVM: nVMX: Rename prepare_vmcs02_*_full to
prepare_vmcs02_*_rare"), prepare_vmcs02_full has been renamed to
prepare_vmcs02_rare.
nested_vmx_merge_msr_bitmap is renamed to nested_vmx_prepare_msr_bitmap
since commit c992384b ("KVM: vmx: speed up MSR bitmap merge").

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>