- mark deviceNode const
- copy it as deviceName
- mutate deviceName by removing /dev/
- use the new deviceName for the read-only check
this makes sure we emit the unmodified deviceNode (e.g. /dev/vda) rather
than the name (e.g. vda). unbreaks for example calamares which doesn't
know what to do with "vda".
CCBUG: 378607