Correct exception args in vfs/guestfs.

Updates the exception formatting in vfs/guestfs so that we
properly raise in cases where there is no root or multiple
roots.

Fixes a TypeError that would occur with the previous code.

Fixes LP Bug #1153414.

Change-Id: I28f9cde5a2da466ebb7397176baaf051ccd4192b
Merge "Correct exception args in vfs/guestfs."