python: Fix incorrect prefixes is m5.utils.convert

python: Fix incorrect prefixes is m5.utils.convert

The conversion functions incorrectly assumed that kibibytes are 'kiB'
rather than 'KiB' (correct).

Change-Id: I7ef9e54546fdb3379435b40af6d9f619ad9b37a5
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39375
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>