pc: use generic cpu_model parsing

define default CPU type in generic way in pc_machine_class_init()
and let common machine code to handle cpu_model parsing

Patch also introduces TARGET_DEFAULT_CPU_TYPE define for 2 purposes:
  * make foo_machine_class_init() look uniform on every target
  * use define in [bsd|linux]-user targets to pick default
    cpu type

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1505318697-77161-5-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>