KVM: x86: handle invalid root_hpa everywhere

commit 37f6a4e2 upstream.

Rom Freiman <rom@stratoscale.com> notes other code paths vulnerable to
bug fixed by 989c6b34.

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>