xen-tools: Reduce QEMU memory consumption

[temerkhanov]

• Reimplement coroutine pool to save memory allocations/deallocations
• Reduce thread stack sizes
• Reimplement mmap-based memory allocation function

Signed-off-by: Sergey Temerkhanov [email protected]

[temerkhanov]

Due to the nature of this PR, I'd like it to be tested on various workloads

[rouming]

[the hostess took up the broom] @eriknordmark can we just close this? this looks like a total mess, original commits are lost after the trunk was diverged, barely possible to figure out what was really changed.