4.4.0-91.114) is not available, but a newer one is (4.4.0-142.168). We redirected you there.
/usr/lib/linux-tools/4.4.0-72-lowlatency/usbipd is in linux-tools-4.4.0-72-lowlatency 4.4.0-72.93~14.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/linux-tools/4.4.0-72-lowlatency/../../linux-lts-xenial-tools-4.4.0-72/usbipd