exec.c: ensure all AddressSpaceDispatch updates under RCU

The memory_dispatch field is meant to be protected by RCU so we should
use the correct primitives when accessing it. This race was flagged up
by the ThreadSanitizer.

Signed-off-by: Alex BennÃ©e <alex.bennee@linaro.org>
Message-Id: <20161021153418.21571-1-alex.bennee@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
