Make "clean" action more robust

[conorsch]

The current dev env supports a make clean action, which attempts to purge SDW-specific configurations. There are a few exceptions, however, which linger after the clean:

• sys-firewall DNF repo customizations (in /rw/config/ and /etc/)
• dom0 RPC policies
• legacy VM names (e.g. sd-export-usb, see Remove sd-export-usb persistent USB device attachment #323 (review))
• qubes-prefs such as default_dispvm (Set sd-svs-disp as default DispVM #312)

In particular, the qubes-prefs issue prevents removal of the sd-svs-disp{,-template} VMs, so we should consider reverting that immediately until this issue is resolved.