Migrate should handle different volume types better

[eschrock]

Problem

Currently migrate requies that the data sources be mounted from the host with -v. If it doesn't, then you get a byzantine error message.

Solution

Either implement a method for pulling data from other volume types, or at least give a nice error message indicating that it's not supported.

[mcred]

Migrate is intended to have multiple mount types, more than just local data, and should be implemented.