Update strings used in RAM/Disk properties

We probably won't have time to implement the interface shown in
https://raw.github.com/gnome-design-team/gnome-mockups/master/boxes/boxes-install5.5.png
for the 3.6 release, but we can at least make sure we use the recommended wording.

https://bugzilla.gnome.org/show_bug.cgi?id=683092