disk: Provide a default storage source format type.

disk: Provide a default storage source format type.

https://bugzilla.redhat.com/show_bug.cgi?id=1181062

According to the formatstorage.html description for <source> element
and "format" attribute: "All drivers are required to have a default
value for this, so it is optional."

As it turns out the disk backend did not choose a default value, so I
added a default of "msdos" if the source type is "unknown" as well as
updating the storage.html backend disk volume driver documentation to
indicate the default format is dos.