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 <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Cc: Anthony Liguori <anthony@codemonkey.ws>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>