path_id: reintroduce by-path links for virtio block devices

Enumeration of virtio buses is global and hence
non-deterministic. However, we are guaranteed there is never going to be
more than one virtio bus per parent PCI device. While populating
ID_PATH we simply skip virtio part of the syspath and we extend the path
using the sysname of the parent PCI device.

With this patch udev creates following by-path links for virtio-blk
device /dev/vda which contains two partitions.

ls -l /dev/disk/by-path/
total 0
lrwxrwxrwx 1 root root  9 Feb  9 10:47 virtio-pci-0000:00:05.0 -> ../../vda
lrwxrwxrwx 1 root root 10 Feb  9 10:47 virtio-pci-0000:00:05.0-part1 -> ../../vda1
lrwxrwxrwx 1 root root 10 Feb  9 10:47 virtio-pci-0000:00:05.0-part2 -> ../../vda2

See:
http://lists.linuxfoundation.org/pipermail/virtualization/2015-August/030328.html

Fixes #2501

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