qdev: hotplug: Introduce HotplugHandler.pre_plug() callback

pre_plug callback is to be called before device.realize() is executed.
This would allow to check/set device's properties from HotplugHandler.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>