coroutine: add qemu_coroutine_yield benchmark

Current coroutine performance benchmarks test only coroutine creation,
either directly or in a nested way. This patch adds a benchmark to
evaluate the performance of qemu_coroutine_yield.

Signed-off-by: Gabriel Kerneis <gabriel@kerneis.info>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
coroutine: fix /perf/nesting coroutine benchmark

The /perf/nesting benchmark is broken because the counters are
not reset after each iteration. Therefore, nesting is done only
on the first iteration, and skipped on every other.

This patch fixes the issue, and reduces the number of iterations
to make it possible to run the benchmark in a reasonable amount of
time.

Signed-off-by: Gabriel Kerneis <gabriel@kerneis.info>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>