qemu-img: Use qemu_strtoul() rather than raw strtoul()

Some of the argument parsing in qemu-img uses strtoul() to parse
integer arguments.  This is tricky to get correct and in fact the
code does not get it right, because it assigns the result of
strtoul() to an 'int' variable and then tries to check for > INT_MAX.
Coverity correctly complains that the comparison is always false.

Rewrite to use qemu_strtoul(), which has a saner convention for
reporting conversion failures.

(Fixes CID 1356421, CID 1356422, CID 1356423.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1486744104-15590-2-git-send-email-peter.maydell@linaro.org
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Max Reitz <mreitz@redhat.com>