qobject: make refcount atomic

qobject: make refcount atomic

The Rust bindings for QObject will only operate on complete objects,
treating them as immutable as long as the Rust QObject is live.

With that constraint, it is trivial for Rust code to treat QObjects as
thread-safe; all that's needed is to make reference count operations
atomic.  Do the same when the C code adds or removes references, since
we don't really know what the Rust code is up to; of course C code will
have to agree with not making changes to the QObjects after they've
been passed to Rust code.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>