Make fsimage comparator handle FAT filesystem

Detecting FAT apparently is tricky, because file type "DOS/MBR" is detected for many other formats too. Do that by manually checking magic value (the specification(?) discourages using this value for anything, but looks like all sane tools create filesystem with this exact signature of "FAT12"/"FAT16"/"FAT32").

There is also an attempt to fix tests (libguestfs require some kernel to be installed). This makes things better for most cases (fsimage test now runs and succeed). But also breaks stable-bpo test, where initramfs-tools installation fails on grub failing to find root device.