Fixes Hyper-V Inspector disk metrics cache issue

Hyper-V Inspector yields a Disk with the property
'device' containing a dictionary. This causes an
exception when the values are cached in a dictionary
using the said 'device' as a key.

Closes-Bug: #1395807

Change-Id: I09b4ec2a37a6f7e6580a059517ab6e606bfc346f
(cherry picked from commit a2e11a37aca6d4c0f76376938a147a37b1950ec6)