I don't know yet what will be the best way to configure the tests, but
this changes the test code to allow both modes to be used: a single
model can be set using the "cpu_model" option, or multiple models may be
tested using the "cpu_models" option. Or, if cpu_models is "*", all
models reported by QEMU can be tested (that was the current behavior
when cpu_models was not set).
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>