tests/migration-test: don't spam the logs when we fail

Quite often the information about which test failed is hidden by the
wall of repeated failures for each page. Stop outputting the error
after 10 bad pages and just summarise the total damage at the end.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>