simg2img has seeking error with input stdin

Tried to create image from stdin sparse image, but got this error. Source code at master branch commit 2807b36

# cat image.img | simg2img - test.ext4 
Unknown error -29 at seeking
Failed to read sparse file

From the code, it looks like the input must be able to seek, but stdin doesn't.
If it doesn't support stdin, just remove this capable in main()?

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