Honor anti-affinity policy on migration and evacuation

If you create a server group with an anti-affinity policy, it is only honored during the initial boot. If you do a cold migrate, live migrate, or evacuate where the scheduler is picking the destination, it seems reasonable to expect that the scheduler will continue to honor that policy. However, it does not.

The reason for this has to do with an implementation optimization in the scheduler. It skips all of the group checking if the 'group' hint is not present. Since scheduler hints are only kept around for the initial boot, this doesn't work. One solution would be to persist scheduler hints. However, a shorter term fix specifically for server groups is to always check the database for group membership when the server group filters are enabled.