Tidy up memory/storage entry widgets
They could be:
- step over more reasonable amounts than https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/master/src/preferences/ram-row.vala#L18
- handle text input -> https://gitlab.gnome.org/GNOME/gnome-boxes/-/blob/master/src/preferences/memory-row.vala#L16