qdev: Fold state enum into bool realized

Whether the device was initialized or not is QOM-level information and
currently unused. Drop it from device. This leaves the boolean state of
whether or not DeviceClass::init was called or not, a.k.a. "realized".

Suggested-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
