hw/qdev-core: Fix description of instance_init

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>