Allow guestfs to modify the rootfs
For x86_64, it works well to pass two devices to the kernel and expect the kernel to load the correct drive using root=UUID=23423423...
With aarch64, this typically fails. This leads to enumeration problems as it is uncertain whether to use /dev/vda or /dev/vdb etc. aarch64 tests also have issues mounting the overlay using /dev/disk/by-uuid/ as QEMU omits the drive when no suitable device is setup.
Investigate using GuestFS to write the overlay to the rootfs specified by the test writer as an option instead of using a second drive. Use the install_overlay support.
- Warn that the image is being modified.
- document that guestfs supports ext2/3/4 and btrfs but other filesystems could cause issues
Migrated from https://projects.linaro.org/browse/LAVA-787