gwtop crashes if iscsi target doesn't expose any luns

```
# gwtop --debug
Traceback (most recent call last):
File "/usr/bin/gwtop", line 282, in
options = get_options()
File "/usr/bin/gwtop", line 162, in get_options
if device_map[id]['lun_type'] == 'user'])
KeyError: 'lun_type'
```

Start with /usr/bin/gwtop and inspect get_options() around line 162, where the traceback accesses device_map[id]['lun_type']. Reproduce the command against an iSCSI target with no exposed LUNs; done means gwtop no longer raises KeyError in that case.

Written by the indexing model from the issue text.