qemu-img: Document check exit codes

qemu-img: Document check exit codes

The exit code 63 (check not supported by image format) was not even
documented in the comment above the check command in the source code;
add it, as it does indeed seem useful.

Also, document all of check's exit codes in the manpage.

Signed-off-by: Max Reitz <[email protected]>
Reported-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Benoit Canet <[email protected]>
Reviewed-by: Kevin Wolf <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>