target-i386: Split out CPU creation and features parsing

Move CPU creation and features parsing into a separate cpu_x86_create()
function, so that board would be able to set board-specific CPU
properties before CPU is realized.

Keep cpu_x86_init() for compatibility with the code that uses cpu_init()
and doesn't need to modify CPU properties.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>