qemu: allow to apply the lava overlay directly on the rootfs

This can be used with this definition:

```yaml
rootfs:
 image_arg: "-drive file={rootfs},if=none,format=raw,id=hd0 -device virtio-blk-device,drive=hd0"
 url: "https://storage.tuxboot.com/arm64/rootfs.ext4.zst"
 compression: zstd
 format: ext4
 overlays:
   lava: true
```

This is needed for some qemu machines that does not have spurious devices for
attaching the lava-guest.qcow2 image.

Signed-off-by: Rémi Duraffort <remi.duraffort@linaro.org>
16 jobs for qemu-lava-overlay in 5 minutes and 27 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
failed #127536
amd64
dispatcher-debian-10

00:02:26

failed #127538
amd64
dispatcher-debian-11

00:02:28

passed #127537
amd64
server-debian-10

00:03:03

passed #127539
amd64
server-debian-11

00:05:25

 
  Analyze
skipped #127540
amd64-dind allowed to fail
bandit-sast
skipped #127541
amd64
black
skipped #127542
amd64
code_quality
skipped #127547
amd64
codespell
skipped #127543
amd64
coverage
skipped #127544
amd64
dockerfiles
skipped #127546
amd64
pylint
skipped #127545
amd64
schemas
 
  Build
skipped #127549
amd64
debian/10
skipped #127548
amd64
doc
skipped #127550
amd64-dind
docker-amd64-dispatcher
skipped #127551
amd64-dind
docker-amd64-server
 
Name Stage Failure
failed
dispatcher-debian-11 Test
FAILED tests/lava_dispatcher/test_kvm.py::TestKVMInlineTestDeploy::test_pipeline
FAILED tests/lava_dispatcher/test_kvm.py::TestKvmConnection::test_kvm_connection
====== 11 failed, 453 passed, 6 skipped, 27 warnings in 127.01s (0:02:07) ======
Uploading artifacts for failed job
Uploading artifacts...
WARNING: coordinator.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1
failed
dispatcher-debian-10 Test

-- Docs: https://docs.pytest.org/en/latest/warnings.html
======= 11 failed, 456 passed, 3 skipped, 29 warnings in 128.22 seconds ========
Uploading artifacts for failed job
Uploading artifacts...
WARNING: coordinator.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1