retrace-client: humanize sizes

G_FORMAT_SIZE_IEC_UNITS

 use IEC (base 1024) units with "KiB"-style suffixes. IEC units should
only be used for reporting things with a strong "power of 2" basis, like
RAM sizes or RAID stripe sizes. Network and storage sizes should be
reported in the normal SI units.

Fixes #844

Signed-off-by: Jakub Filak <jfilak@redhat.com>