[RFC] make VM handling async

[Christian König]

Yeah, I know I'm on vacation, but without coding I die of boringness in less
than a week.

This patchset is currently based on Jeromes VA fencing patch, but in the end
makes it superfluous. It just moves the whole VM handling into the CP ring,
removing the need to wait for anything directly in the VM code, the any waiting
that happens is now in the allocation of page tables and that happens only on
allocation of new VM space and not on freeing it, which in turn makes handling
allot more simpler.

It's not 100% complete cause page table updates should be made with the DMA
ring, but we haven't released documentation for that yet, so I currently
use CP memory writes instead.

Please take a look and tell me what you think about it.

Cheers,
Christian.