[Libguestfs] [PATCH nbdkit] server: Move size parsing code (nbdkit_parse_size) to common/include

This is the first part of a pair of patch series which aim to let us use nbdkit_parse_size (or rather, an equivalent common function) in nbdcopy, so we can write: nbdcopy --request-size=32M ... We can't do that now which was annoying me earlier in the week. This commit creates a new function called human_size_parse which is basically nbdkit_parse_size, and turns nbdkit_parse_size into a wrapper around it. Rich.