fix #143: add disk/mem percent sort

fix #143: add disk/mem percent sort

previously, we had a disk/memory usage column in the resource grid,
whose raw data were the bytes used, but we added a renderer
to display it as percentage

with this, the columns sorted by bytes and not by percentage,
which made the column rather confusing

with this patch, we add a real percentage column (where the
data is a float from 0 to 1) so it sorts correctly

also make the old columns show the used size in B/KiB/etc.
by default we still only show the percentage column

since the disk usage part was always zero with qemu guests,
leave it empty there

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>