With the XenCenter update installation tool, it is no longer possible to choose individual hosts within a pool to apply the hotfix(es) to. This, while designed to be helpful, is actually a hindrance in some cases where you want to deliberately apply the hotfix to a selected host or subset of hosts. Now, it appears one has to resort to the CLI to accomplish this:
xe upload-apply host-uuid=<UUID_of_host> uuid=<UUID_of_file>
There are times it would be much more convenient to apply all updates and choose to manually do a reboot or toolstack restart at a later time.