pseries: Add spapr_rtc_read() helper function

pseries: Add spapr_rtc_read() helper function

The virtual RTC time is used in two places in the pseries machine.  First
is in the RTAS get-time-of-day function which returns the RTC time to the
guest.  Second is in the spapr events code which is used to timestamp
event messages from the hypervisor to the guest.

Currently both call qemu_get_timedate() directly, but we want to change
that so we can properly handle the various -rtc options.  In preparation,
create a helper function to return the virtual RTC time.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>