hw/qdev-core: Fix description of instance_init

The part of the documentation of DeviceClass that talks about instance_init
is partly wrong: instance_init() functions must not abort or exit, since
the function is also called during introspection of the device already.
So if a device calls exit() during its instance_init() function, QEMU
terminates unexpectedly if somebody tries to just have a look at the
interfaces from the device with "device_add xyz,help" or with the
"device-list-properties" QOM command. This should never happen.

Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

include/hw/qdev-core.h

@@ -51,8 +51,9 @@ struct VMStateDescription;
  * Devices are constructed in two stages,
  * 1) object instantiation via object_initialize() and
  * 2) device realization via #DeviceState:realized property.
- * The former may not fail (it might assert or exit), the latter may return
- * error information to the caller and must be re-entrant.
+ * The former may not fail (and must not abort or exit, since it is called
+ * during device introspection already), and the latter may return error
+ * information to the caller and must be re-entrant.
  * Trivial field initializations should go into #TypeInfo.instance_init.
  * Operations depending on @props static properties should go into @realize.
  * After successful realization, setting static properties will fail.