The fast path of cpu_physical_memory_sync_dirty_bitmap() directly
manipulates the dirty bitmap.  Use atomic_xchg() to make the
test-and-clear atomic.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1417519399-3166-7-git-send-email-stefanha@redhat.com>
[Only do xchg on nonzero words. - Paolo]
Reviewed-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>