block/sheepdog: Convert from DPRINTF() macro to trace events

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181213162727.17438-5-lvivier@redhat.com
[mreitz: Fixed sheepdog_snapshot_create_inode's format string to use
         PRIx32 for uint32_ts]
Signed-off-by: Max Reitz <mreitz@redhat.com>