build: Allow building the disk image without root access

[alexandruavadanii]

The current build implementation leverages kpartx and loop devices to copy the rootfs contents to the final disk image, together with a manual step of determining the loop device index.
Instead, we could use FUSE and/or libguestfstools and have the build process work strictly in userspace.
This would add another build dependency.
Also, I'm not sure how this would work with grub-install (I hope it will just work), since I previously had issues with grub-install on loop device partitions and had to add another level of abstraction using nbd.
I'll try to investigate this myself and report here anything useful that I find.

[joshiemoore]

See #98