The mc146818rtc driver exposes the current RTC date and time via the "date"
property in QOM (which is also aliased to the machine's "rtc-time"
property).  Currently it uses a custom visitor function rtc_get_date to
do this.

This patch introduces new helpers to the QOM core to expose struct tm
valued properties via a getter function, so that this functionality can be
more easily duplicated in other RTC implementations.
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NAlexander Graf <agraf@suse.de>