Trace at memory_region_sync_dirty_bitmap() for log_sync() or global_log_sync()
on memory regions.  One trace line should suffice when it finishes, so as to
estimate the time used for each log sync process.

Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20210817013706.30986-1-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>