qdev: Fix QOM unrealize behavior

Since commit 249d41720b7dfbb5951b430b9eefdbee7464f515 (qdev: Prepare
"realized" property) setting realized = true would register the device's
VMStateDescription, but realized = false would not unregister it. Fix that.

Moving the code from unparenting also revealed that we were calling
DeviceClass::init through DeviceClass::realize as interim solution but
DeviceClass::exit still at unparenting time with a realized check.
Make this symmetrical by implementing DeviceClass::unrealize to call it,
while we're setting realized = false in the unparenting path.
The only other unrealize user is mac_nvram, which can safely override it.

Thus, mark DeviceClass::exit as obsolete, new devices should implement
DeviceClass::unrealize instead.

Cc: qemu-stable@nongnu.org
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Message-id: 1366043650-9719-1-git-send-email-afaerber@suse.de
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
