Fixed 'Hourly' charts for VMs that are grouped using tags

Previously, the hourly charts always showed 'None' as in no records
found when the 'Group by' option was selected to retrieve VMs
associated with a particular tag, even though the records did exist.
This was due to the fact that the model name for 'association_type'
that was used to retrieve the records was 'Vm' instead of 'VmOrTemplate'

https://bugzilla.redhat.com/show_bug.cgi?id=1009951