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 systemd#2501

Cherry-picked from: f073b1b
Resolves: #952567

src/udev/udev-builtin-path_id.c

@@ -698,6 +698,12 @@ static int builtin_path_id(struct udev_device *dev, int argc, char *argv[], bool
                         path_prepend(&path, "xen-%s", udev_device_get_sysname(parent));
                         parent = skip_subsystem(parent, "xen");
                         supported_parent = true;
+                } else if (streq(subsys, "virtio")) {
+                        while (parent && streq_ptr("virtio", udev_device_get_subsystem(parent)))
+                                parent = udev_device_get_parent(parent);
+                        path_prepend(&path, "virtio-pci-%s", udev_device_get_sysname(parent));
+                        supported_transport = true;
+                        supported_parent = true;
                 } else if (streq(subsys, "scm")) {
                         path_prepend(&path, "scm-%s", udev_device_get_sysname(parent));
                         parent = skip_subsystem(parent, "scm");