xenserver: Replace customized xen-bugtool with plugin to collect qdisc info.

xenserver: Replace customized xen-bugtool with plugin to collect qdisc info.

XenServer RPMs have installed a modified /usr/sbin/xen-bugtool that is
identical to upstream except for an additional command to collect tc qdisc
class configuration for each network interface. Since xen-bugtool has an
extension mechanism, use that instead of maintaining a parallel patched
xen-bugtool.

This does change the content of the bug report slightly. Instead of one file
per network interface, qdisc class info for all interfaces is written to one
file. This is an artifact of the xen-bugtool extension mechanism: a single
extension command's output must go to one file, and it is impractical to create
individual <command> tags for each interface dynamically.

Reviewed by Ben Pfaff.