Is there some reason why you didn't use davidfor's suggested svg code quoted in your post (which BTW is also how calibre codes svg cover images) i.e.
Code:
<body>
<div>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
width="100%" height="100%" viewBox="0 0 970 1360"
preserveAspectRatio="xMidYMid meet">
<image width="970" height="1360" xlink:href="../Images/Silencio-de-Blanca.jpg"/>
</svg>
</div>
</body>
Added: You need to wrap <svg> code in <div> tags...
Added2: ... or maybe it's only Sigil which flags an error if you don't.
At least, this seems to work OK for standard epubs on Kobo fw3.2.0. but I don't know whether it also works in kepubs.