CL_MIGRATE_MEM_OBJECT_HOST is not implemented

[BrannonKing]

You're supposed to be able to use CL_MIGRATE_MEM_OBJECT_HOST with clEnqueueMigrateMemObjects to reverse the copy direction, to make the copy go device-to-host. However, it's not actually implemented in this code stack. The flag is not used to reverse the copy direction. The flag exists, and a unit test verifies that it doesn't throw an exception when used (but it doesn't verify the data copy). You can see that the flag (and flags parameter) is unused in clEnqueueMigrateMemObjectsIntelFPGA.