Until now we've tried to report errors from the test monitor code by
passing them back as failures from the qemu we simulate. This doesn't
work well in cases when the monitor logic does not detect failures or
has fallback code. Additionally there isn't much use for continuing the
test execution after first failure as in most cases the test data will
be misaligned and all other calls will fail as well.

To make the errors more obvious this patch moves away from reporting
them via the simulated monitor to reporting them to stderr and
exit()ing afterwards. While this might be less convenient
when developing tests it actually makes failures in the test suite
really obvious and doesn't require any opt-in from the tests themselves.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>