Optimization fix in {resource}_count plugin_db APIs

When there are no filters or just tenant-id filter for {resource}_count,
it is better to get the count of the resources directly from the
vnc library instead of calculating the len() return values of {resource}_list call.

Fixes bug: #1391801

Change-Id: I5d80a5e1df5f752b41cb1adcc7e8e989fcf83548