pci: Convert core to realize

Implement DeviceClass methods realize() and unrealize() instead of
init() and exit().  The core's initialization errors now get
propagated properly, and QMP sends them instead of an unspecific
"Device initialization failed" error.  Unrealize can't fail, so no
change there.

PCIDeviceClass is unchanged: it still provides init() and exit().
Therefore, device models' errors are still not propagated.

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>


