CLI should not try to parse more than one format file

[tdaede]

This results in this rather obtuse error:

[libprotobuf ERROR google/protobuf/text_format.cc:309] Error parsing text-format ConfigProto: 1:1: Expected identifier, got: `

if an image file is specified without the -i flag.

[davidgiven]

This is actually intentional. It allows multiple presets to be applied, one after the other. There's actually a PR in progress to use it for the Micropolis format to apply configuration settings needed for a particular disk format. I'll agree the error's not very helpful, however. It may be sensible to try and do a simple test for a text file.