cirrus: don't run full qtest on macOS

cirrus: don't run full qtest on macOS

The Cirrus CI macOS build hosts have exhibited a serious performance
degradation in recent months. For example the "qom-test" qtest takes
over an hour for only the qemu-system-aarch64 binary. This is as much
20-40 times slower than other environments. The other qtests all show
similar performance degradation, as do many of the unit tests.

This does not appear related to QEMU code changes, since older git
commits which were known to fully complete in less than 1 hour on
Cirrus CI now also show similar bad performance. Either Cirrus CI
performance has degraded, or an change in its environment has exposed
a latent bug widely affecting all of QEMU. Debugging the qom-test
showed no easily identified large bottleneck - every step of the
test execution was simply slower.

macOS builds/tests run outside Cirrus CI show normal performance.

With an inability to identify any obvious problem, the only viable
way to get a reliably completing Cirrus CI macOS job is to cut out
almost all of the qtests. We choose to run the x86_64 target only,
since that has very few machine types and thus is least badly
impacted in the qom-test execution.

With this change, the macOS jobs complete in approx 35 minutes.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210106114159.981538-1-berrange@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>