qapi: fail hard on stack imbalance QmpOutputVisitor will segfault if an imbalanced end function is called. So we can abort in QmpInputVisitor too. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Anthony Liguori <aliguori@us.ibm.com> Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>