svg图片填充circle

<svg width="100" height="100">
  <defs>
    <pattern id="image" patternUnits="userSpaceOnUse" height="100" width="100">
      <image x="0" y="0" height="100" width="100" xlink:href="http://i.imgur.com/7Nlcay7.jpg"></image>
    </pattern>
  </defs>
  <circle id='top' cx="50" cy="50" r="50" fill="url(#image)"/>
</svg>