Improve QEMU debugging

[pothos]

• platform/machine/qemu: Log IP address in debug mode
• platform/machine/unprivqemu: Log localhost port for SSH connections
• Log PIDs of qemu machine processes for manual cleanup
• kola: Remove unnecessary deletion of test machines
  The cluster deletes all machines if needed. Deleting them inside the
  tests broke the --remove=false flag to keep machines for debugging.

How to use/testing done

# --key PATH is needed because we run as root
sudo ./kola run -d --remove=false --key ~/.ssh/id_rsa.pub -k --board=amd64-usr --channel=alpha --parallel=1 --platform=qemu --qemu-bios=bios-256k.bin --qemu-image flatcar_production_qemu_image.img systemd.journal.remote 2> >(tee pidlog >&2)
# See test failing, enter with SSH:
ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -o ProxyCommand="sudo nsenter -n -t $(grep 'dnsmasq.*manual cleanup needed' pidlog | cut -d : -f 5) nc %h %p" -p 22 [email protected]
# Later kill the qemu and dnsmasq processes:
sudo kill $(grep 'manual cleanup needed' pidlog | cut -d : -f 5)

[sayanchowdhury]

LGTM