CA-148245: Fix the error where XenCenter did not refresh immediately after disconnecting a server

Added a RequestRefreshTreeView on ClearingCache event.

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
Merge pull request #243 from MihaelaStoica/CA-148245

CA-148245: Fix the error where XenCenter did not refresh immediately after disconnecting a server