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
Rewrap two lines.

No functional changes. This just makes grepping for qemu-img calls a bit easier.

Change-Id: Ifa91a981975bc3766a0f701ee72e5718ff4bec68
Merge "Correct exception args in vfs/guestfs."