CA-141867: 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 canonical backend path for a device is listed in the frontend configuration,
so grab it from there instead of assuming it's at /local/domain/0/device/vbd.