Reuse NIC information from export

If the user doesn't pass any nics in import, do not use a default
one-nic, but instead read the nics from the export file as is.

Fortunately the export and the way nics are read from the command line
are compatible…

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>