As some of you have experienced image mounting failures I believe I have found a possible cause. I thought it might have been a kernel issue, but it seems to be related to btrfs. After switching from btrfs (screwed me over twice) to ext4 as the storage media, image mounting now works perfectly.
When using btrfs, I could only get the image to mount if I ran “sudo libguestfs-test-tool” first. Even then it threw errors, but did eventually mount the image.