I can see the bug right there. The aspect ratio is wrong because the resolution is set to 600x800. So you get your image set to fit as though it was a 600x800 resolution image. When I change 600 and 800 to the actual resolution of the cover image, then I get the correct aspect ratio.
Code:
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" width="100%" height="100%" viewBox="0 0 600 800" preserveAspectRatio="xMidYMid meet">
<image width="600" height="800" xlink:href="cover.jpeg"/>