virtio-pci: Check if is_avq is NULL

[bug]
In the virtio_pci_common.c function vp_del_vqs, vp_dev->is_avq is involved
to determine whether it is admin virtqueue, but this function vp_dev->is_avq
 may be empty. For installations, virtio_pci_legacy does not assign a value
 to vp_dev->is_avq.

[fix]
Check whether it is vp_dev->is_avq before use.

[test]
Test with virsh Attach device
Before this patch, the following command would crash the guest system

After applying the patch, everything seems to be working fine.

Signed-off-by: Li Zhang <zhanglikernel@gmail.com>
Message-Id: <1710566754-3532-1-git-send-email-zhanglikernel@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>