CP-8474: Remove iSL radio button from XenCenter for >= 6.5 servers.

Made the radio button invisible for 6.5 (Creedence) or higher XS hosts.
Removed previous StorageLink-related upgrade pre-checks and added (strict) check to prevent upgrade when StorageLink is used.

Signed-off-by: Gabor Apati-Nagy <gabor.apati-nagy@citrix.com>