Fix datastore listing on vsphere

vmware datastore listing was recently changed to ignore inaccessible
datastores. In fact this was not the problem, CFME does not show up a
datastore on a provider if it is not mounted by any hosts.

This commit changes the datastore listing function to mimic that
behaviour by checking if the datastore is mounted on any hosts.
Merge pull request #1373 from psav/fix_datastore

Fix datastore listing on vsphere