cpu: Initialize breakpoint/watchpoint lists in cpu_common_initfn()

One small step in the simplification of cpu_exec_init().

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