pseries: Make the PAPR RTC a qdev device

pseries: Make the PAPR RTC a qdev device

At present the PAPR RTC isn't a "device" as such - it's accessed only via
firmware/hypervisor calls, and is handled in the sPAPR core code.  This
becomes inconvenient as we extend it in various ways.

This patch makes the PAPR RTC a separate device in the qemu device model.

For now, the only piece of device state - the rtc_offset - is still kept in
the global sPAPREnvironment structure.  That's clearly wrong, but leaving
it to be fixed in a following patch makes for a clearer separation between
the internal re-organization of the device, and the behavioural changes
(because the migration stream format needs to change slightly when the
offset is moved into the device's own state).

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