pci: Permit incremental conversion of device models to realize

pci: Permit incremental conversion of device models to realize

Call the new PCIDeviceClass method realize().  Default it to
pci_default_realize(), which calls old method init().

To convert a device model, make it implement realize() rather than
init().

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>