I attached screenshots to show you how it looks on my monitor, the aspect ratio is preserved, centering the image is easy via CSS rules.
What happens if you remove the width? (part in red)
Code:
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMaxYMid" version="1.1" viewBox="0 0 720 960" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink">
<image height="100%" width="100%" xlink:href="../Images/cover.jpg"></image>
</svg>