Code review comment for ~mwhudson/curtin:lp-1893818

Thanks for the comments, some replies to the inline questions.

Can you look at https://code.launchpad.net/~mwhudson/curtin/+git/curtin/+merge/396462 first?

I changed the ProbertParser.is_mpath_* methods to just forward to the multipath.is_mpath_* functions, which meant I had to change one of the latter to work via udev. We don't need any of this knowledge even more spread around that it was already...