Add getHostPtr() to vk::Allocation

Add getHostPtr() to vk::Allocation

Since there can exist only one memory mapping for any VkDeviceMemory
and vkUmapMemory() unmaps all pages, current Allocator interface doesn't
extend to sub-allocators properly. To make sub-allocation work,
Allocator must be responsible of managing memory mappings.

The new behavior is to always map any host-visible allocations and
provide host pointer as part of Allocation.

Change-Id: I83a28b1387b282a373604cb112757ffdd75b20b3