virtio-pci: decouple notifier from interrupt process

To reuse the notifier process in configure interrupt.
Use the virtio_pci_get_notifier function to get the notifier.
the INPUT of this function is the IDX, the OUTPUT is notifier and
the vector

Signed-off-by: Cindy Lu <lulu@redhat.com>
Message-Id: <20211104164827.21911-3-lulu@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>