htools: add parseUnit function

This is similar, but not identical, to Utils.ParseUnit. The biggest
difference is that we don't round up/down; as we only use integral
types, the result will always be rounded down.

Moreover, since (real-world) disk sizes come in SI units, the function
differentiates between SI and binary prefixes, using lower-case for
binary and upper-case for SI (similar to lvm usage). This distinction
should be ported to the Python code.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>