"virGetDeviceID" could be used across the sources, but it doesn't
relate with this series, and could be done later.

* src/util/virutil.h: (Declare virGetDeviceID, and
                       vir{Get,Set}DeviceUnprivSGIO)
* src/util/virutil.c: (Implement virGetDeviceID and
                       vir{Get,Set}DeviceUnprivSGIO)
* src/libvirt_private.syms: Export private symbols of upper helpers