initialize CPU model list after handling -readconfig options

To properly load cpudefs using -readconfig, we have to call
cpudef_init() after finishing the command-line option handling.

Consequently, the handling of "-cpu ?" has to be done after the
command-line option handling loop, too.

Without this patch, "-readconfig configfile -cpu ?" fails to list the
CPU definitions read from 'configfile'.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
