CA-114990: XenCenter should not pop up the license expiry warning for Clearwater and later hosts

- The License Manager dialog is no longer shown when connecting to unlicensed Clearwater and later hosts; same rule applies for periodic checks;
- Minor refactoring: combined two functions that did the same thing (CheckActiveServerLicense and periodicCheck) into one.
Merge branch 'master' of git://hg.uk.xensource.com/carbon/trunk/xenadmin