virt-lxc: attempt to cleanly shutdown the container first, and only then pass...

virt-lxc: attempt to cleanly shutdown the container first, and only then pass --force to lxc-destroy

Passing --kill to the initial lxc-stop call results in the associated DHCP lease never being released to dnsmasq in a typical setup.

On worker hosts that are not ephemeral cloud instances, this quickly depletes the standard DHCP pool: autopkgtests running after that happens will all tmpfail since they won't be able to get an IP address at all.