Truncate long RHEV custom attributes

Using the RHEV DirectLUN hook in support for CFME SmartState
Analysis, it is very easy to have a RHEV custom attribute
which exceeds the 255 character limit.  This prevents the
affected VMs from being updated and also prevents deleted RHEV
VMs from being being flagged as deleted in the VMDB. Updated the
RHEV inventory parser to truncate custom attribute values that
exceed the DB limit.

https://bugzilla.redhat.com/show_bug.cgi?id=1122259