io: assert errors before asserting content in I/O test

When checking the results of an I/O operation test, assert that
the error objects are NULL before asserting on the content. This
is found to give more useful indication of the problem when
diagnosing test failures.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
