CA-141867 (v2): backend paths may be either /local/domain/0/device/vbd or vbd3

Currently, xcp-rrdd-iostat assumes the backend configuration is in /local/domain/0/device/vbd, but this is wrong for VBDs that use tapdisk3.

The first approach I took (https://github.com/xenserver/rrdd-plugins/pull/9) obtained the backend path from the frontend configuration, but this was susceptible to attack by malicious guests. A much easier option is just to try both vbd and vbd3 in the backend path. This pull request adopts that approach.