tests/qtest: Adjust and document query-cpu-model-expansion test for arm

tests/qtest: Adjust and document query-cpu-model-expansion test for arm

We're about to move the 32-bit CPUs under CONFIG_TCG, so adjust the
query-cpu-model-expansion test to check against the cortex-a7, which
is already under CONFIG_TCG. That allows the next patch to contain
only code movement. (All the test cares about is that the CPU type
it's checking is one which definitely doesn't work under KVM.)

While here add comments clarifying what we're testing.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-id: 20230426180013.14814-7-farosas@suse.de
Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>