Report error of qemu instead of connection closed
When a problem appears in a qemu job, the job generally fail with "Infrastructure error: Connection closed". But qemu always issue an error as its last line of log. Example:
- qemu-system-ppc64: OS terminated: OS panic: VFS: Unable to mount root fs on unknown-block(1,0)
- qemu-system-aarch64: can't apply global max-arm-cpu.pauth-impdef=on: Property '.pauth-impdef' not found
So LAVA should store the last line of a qemu output and report as "error_msg" of infra error.