This splits out the lorax-composer specific execution so that the built
image can be downloaded from the build vm and booted by the host instead
of using nested virt to try and boot it inside the build vm.
Also adds copying the ssh key from the build vm so that it can log into
the image and run the test_boot_* scripts.
Cherry-picked from df7a018ee2
Related: rhbz#1769525
- save compose logs under /var/log/$TEST
- save qemu logs under /var/log/$TEST/qemu.log
- download everything to $TEST_ATTACHMENTS so it can be saved
in CI results
Cherry-picked from 8af9723c
Related: rhbz#1769525
