fix #1629: improve min/max memory handling

fix #1629: improve min/max memory handling

instead of directly setting the minvalue when the maxvalue is lower
we invalidate it and only on blur we set the minvalue

without this we lose the minvalue when editing only the maxvalue

e.g.:

Max: 2048
Min: 1024

if we now edit the maxvalue to 4096, on pressing the '4', we are
setting the minvalue to '4' because 4 is lower than 1024

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