ci: build and run a QEMU guest VM for testing

ci: build and run a QEMU guest VM for testing

Since GitHub Actions' free runners now support nested virtualization,
let's create a guest VM image and use it to run some tests that cannot
be run directly via a GitHub Actions runner (because these runners do
not allow insertion of kernel modules).

This patch adds a workflow that runs nightly and:
- creates a Debian guest VM image using libguestfs
- builds QEMU
- starts the VM
- clones/builds fio on the VM
- runs the test

Separate instances of the job are spawned to run the following tests:
- basic io_uring_cmd
- NVMe protection information w/16-bit Guard PI
- NVMe PI 4096+16 w/64-bit Guard PI
- NVMe PI 4096+64 w/64-bit Guard PI
- FDP
- t/zbd/run-tests-against-nullb

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>