Add virtio-blk support to path_id

Add virtio-blk support to path_id

This patch adds a case handling path_id invoked on a virtio-blk device.
Currently path_id walks the parent path to virtio-pci but doesn't know
that it's the end of the path and exits without building the path (providing no
output resulting in no disk/by-path symlinks to virtio-blk devices).
This patch handles the virtio-pci path and updates the path accordingly.

/lib/udev/path_id --debug /block/vda
udev_device_new_from_syspath: device 0x2300120 has devpath '/devices/virtio-pci/virtio1/block/vda'
udev_device_new_from_syspath: device 0x2300380 has devpath '/devices/virtio-pci/virtio1'
udev_device_new_from_syspath: device 0x2300670 has devpath '/devices/virtio-pci'
ID_PATH=virtio-pci-virtio1

And with the current persistent-storage rules generates:

% ls -al /dev/disk/by-path | grep vda
lrwxrwxrwx. 1 root root   9 Jun  1 22:09 virtio-pci-virtio1 -> ../../vda

Signed-off-by: Ryan Harper <ryanh@us.ibm.com>