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.