With this series I can boot a busybox image and several Ubuntu images
with various -smp's.

The speedup we get is in line with what we get with other ISAs, e.g.
-smp 4 takes 34s instead of 1min with thread=single.

I've run this through the Valgrind race detectors, and the remaining
races reported seem benign.

I have not tested i386, so I'm not turning mttcg on for it yet.

Patches 1-2 comply with cpu_get_ticks' documentation. I'm not too
happy about effectively serializing rdtsc, since Linux guests call it
very often (for each vCPU!). But as usual, correctness goes before
performance or scalability.

You can fetch the series from:
  https://github.com/cota/qemu/tree/x86_64-mttcg

Thanks,

		Emilio