CA-147453: xsifstat: Remove concept of netback group

Since kernel c/s b3f980bd827e6e81a050c518d60ed7811a83061d, netback no longer
supports multiple VIFs per kthread. Hence it is redundant for xsifstat to group
VIFs by thread.
CA-147822: xsifstat: Don't assume first line of uevent file is INTERFACE