We do not trace guest atomic accesses. Fix it.

Tested with a modified atomic_add-bench so that it executes
a deterministic number of instructions, i.e. fixed seeding,
no threading and fixed number of loop iterations instead
of running for a certain time.

Before:
- With parallel_cpus = false (no clone syscall so it is never set to true):
  220070 memory accesses
- With parallel_cpus = true (hard-coded):
  212105 memory accesses <-- we're not tracing the atomics!

After:
  220070 memory accesses regardless of parallel_cpus.
Signed-off-by: NEmilio G. Cota <cota@braap.org>
Message-id: 1527028012-21888-6-git-send-email-cota@braap.org
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>