webadmin: Make all disk size setting in DiskModel type safe

Since the size entity in DiskModel is defined as an Integer entity model,
all the places that set the new entity should be in Integer type to ensure
type safety

Change-Id: I2f22ca6865fc7927eb9777986b1844e3b8639141
Signed-off-by: Tal Nisan <tnisan@redhat.com>