nbdtab and systemd nbd@.service are broken in 3.27.0

[bauen1]

Since 3.27.0 the systemd service no longer works, since e.g. nbd-client nbd0 no longer works.

However, using nbd-client /dev/nbd0 will result in get_from_config "stripping" the /dev/ prefix. However nbd-client will later check that this prefix is present and error if it is not:

sysadmin@host:~$ sudo nbd-client /dev/nbd2
Negotiation: ..size = 393216MB
Error: Invalid nbd device target

Exiting.

[bauen1]

I believe this may be fixed by #179 but I am unsure if this MR won't break things like nbd-client /dev/nbd0