memory management improvements

[anttikantee]

There are essentially three parties allocating memory in our DomU:

1. the application (or applications)
2. mini-os
3. rump kernel

Generally speaking, only "3" knows how to deal with memory shortage (some special cases of "1" might too). On the flipside, "3" also will attempt to use all available memory in the current configuration.

One possibility is to let "3" continue to use all the available memory, and hook up the memory allocators for "1" and "2" into trying to make "3" release excess caches upon allocation failure.

Another possibility is to limit the memory "3" will use and apply different allocation zones for each of 1,2,3.