qdev: Prepare "realized" property

qdev: Prepare "realized" property

Introduce the QOM realizefn suggested by Anthony.
Detailed documentation is supplied in the qdev header.

For now this implements a default DeviceClass::realize callback that
just wraps DeviceClass::init, which it deprecates.
Once all devices have been converted to DeviceClass::realize,
DeviceClass::init is to be removed.

Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Andreas Färber <[email protected]>
Cc: Anthony Liguori <[email protected]>
Signed-off-by: Anthony Liguori <[email protected]>