cpu: Rename parse_cpu_model() to parse_cpu_option()

The "model[,option...]" string parsed by the function is not just
a CPU model.  Rename the function and its argument to indicate it
expects the full "-cpu" option to be provided.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190417025944.16154-2-ehabkost@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>