Simplify 'dma heap' logic.

Simplify 'dma heap' logic.

1. Only useful for NUMA systems, so turn it off on non-NUMA systems by
   default.
2. On NUMA systems, by default relate the DMA heap size to NUMA node 0
   memory size (so that not all of node 0's memory ends up being 'DMA
   heap').
3. Remove the 'dma emergency pool'. It's less useful now that running
   out of low memory isn;t as fatal as it used to be (e.g., when we
   needed to be able to allocate low-memory PAE page directories).

Based on patches by Andre Przywara and Xiaowei Yang.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>