The value of 'hrDeviceIndex' corresponding to the first
associated physical device row in the 'hrDeviceTable' in the
Host Resources MIB (RFC 2790) and an (optional) associated row
in 'xcmHrDevInfoTable' of the XCMI Ext to Host Resources MIB,
or zero if none.
Usage: Conforming implementations SHALL ensure that
physical devices underlying services are 'visible'.