Acceptance test x86_cpu_model_versions: split into smaller tests

Acceptance test x86_cpu_model_versions: split into smaller tests

The justifications being automatic destruction of the vm instances
when no longer needed and more compact test naming under a common
class.

Besides those, a smaller test makes the one and only assertion rather
obvious, which suggests that we could even get rid of the more verbose
(and manual) error messages (to be decided).

Naming of the tests tries to follow the following pattern:

 test_($cpu_version)_($no_arch_capabitilies_set_or_unset)_($machine_version)

The presence of each naming component is optional, depending on
whether the test manually sets it or not.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20190828193628.7687-4-crosa@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>