atomics: eliminate mb_read/mb_set

qatomic_mb_read and qatomic_mb_set were the very first atomic primitives
introduced for QEMU; their semantics are unclear and they provide a false
sense of safety.

The last use of qatomic_mb_read() has been removed, so delete it.
qatomic_mb_set() instead can survive as an optimized
qatomic_set()+smp_mb(), similar to Linux's smp_store_mb(), but
rename it to qatomic_set_mb() to match the order of the two
operations.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

monitor/qmp.c

@@ -246,7 +246,7 @@ static QMPRequest *monitor_qmp_dispatcher_pop_any(void)
          *
          * Clear qmp_dispatcher_co_busy before reading request.
          */
-        qatomic_mb_set(&qmp_dispatcher_co_busy, false);
+        qatomic_set_mb(&qmp_dispatcher_co_busy, false);
 
         WITH_QEMU_LOCK_GUARD(&monitor_lock) {
             QMPRequest *req_obj;