target-i386: Rename cpu_x86_register() to x86_cpu_load_def()

There isn't any kind of "registration" involved in cpu_x86_register()
anymore: it is simply looking up a CPU model name and loading the model
definition data into the X86CPU object. Rename it to x86_cpu_load_def()
to reflect what it does.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>