qapi: add struct-errors test case to test-qmp-output-visitor

This test case verifies that invalid native enums are caught, and causes
qapi to tear down the QObject tree under construction, exercising the
previous patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
