Checkbox does not support the PCI_CLASS reported by some virtio-pci devices

Some virtio-pci devices report the following virtual PCI information:

P: /devices/pci0000:00/0000:00:06.0
E: UDEV_LOG=3
E: DEVPATH=/devices/pci0000:00/0000:00:06.0
E: DRIVER=virtio-pci
E: PCI_CLASS=FF00
E: PCI_ID=1AF4:1003
E: PCI_SUBSYS_ID=1AF4:0003
E: PCI_SLOT_NAME=0000:00:06.0
E: MODALIAS=pci:v00001AF4d00001003sv00001AF4sd00000003bc00scFFi00
E: SUBSYSTEM=pci

The problem here is that physical PCI information always contains prog_if information as part of the PCI_CLASS, so the value is assumed to contain more than 4 hexadecimal characters. However, in the above example, this information is not available so strictly 4 characters are provided. This is causing the udev registry to fail when parsing the output of udevadm.