CA-142255:XenCenter popup exception dialog which is unexpected

This problem happens because XenCenter maybe lost connection some times and the operation check here will use ValidRoleList check before connection check, which will cause the roles in connection be empty and generate that exception when connection is not connected.
Check connection status before get ValidRoleList will solve this problem.

Signed-off-by: Cheng Zhang <cheng.zhang@citrix.com>