ui: use byte unit and power-of-two base for memory rrd graphs

ui: use byte unit and power-of-two base for memory rrd graphs

To better match the gauge in the current status infobox in the same
summary panel, which is already using a format where KiB is 1024

the latter needs a (not yet bumped) widget toolkit update to 2.5-2 or
newer, just setting now to avoid forgetting it.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>