Human-readable serial number, used by system administrators
and end users to identify this device for systems management.
Usage: Conforming management agents, which ALSO implement the
the Printer MIB v2, SHALL set 'xcmHrDevInfoSerialNumber'
to the SAME value as 'prtGeneralSerialNumber' for the SAME
'hrDevicePrinter' row (i.e., the values of these two objects
SHALL be interlocked).