virtio-pci: Check if is_avq is NULL

In the Linux kernel, the following vulnerability has been resolved:

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.