Add VmHeapArray class to reduce the pressure on VA reservation, since multiple memory pools can be active at the same time.
Null stream isn't created during the device creation
Add initial implementation of virtual memory heap with dynamic virtual memory mapping support for memory pools. DEBUG_HIP_MEM_POOL_VMHEAP controls the new method. Change-Id: I8dc5be2e0f34ab472f1800f43bb6243639a5e500