Don't print None value as 'None' string, but as empty one (same as at
API level). Otherwise it is indistinguishable from VM named 'None', or
same string property value.
This may be confiusing, for example one may think that
`qvm-prefs --unset vmname netvm` will make vmname network-disconnected.
This type of mistakes may have severe security consequence, so better
drop those option names.
QubesOS/qubes-issues#3002
cc @rootkovska
