nspawn: correctly parse a list of interfaces

nspawn: correctly parse a list of interfaces

Interface=/MACVLAN=/IPVLAN= nspawn options take a _list_ of interface
names - this was recently enhanced by 2f091b1b49 to support interface
pairs. Unfortunately, this also introduced a regression where we don't
parse the list as a list, but just as a single value. For example,
having `Interface=sd-shared1 sd-shared2` in an nspawn config file would
throw:

systemd-nspawn[898]: Network interface, interface name not valid: sd-shared1 sd-shared2
systemd-nspawn[898]: /run/systemd/nspawn/testsuite-13.nspawn-settings.1po.nspawn:41: Failed to parse file: Invalid argument

Follow-up to 2f091b1b49.