cmd/incusd: add logs volume config

[c24-tj]

This PR implements #2484

[stgraber]

Logic for setting and handling of the new config key looks good but I think this is missing logic to actually have the instances write their logs to that path.

Most systems run with the default of using /var/log/incus/ so having a path now ready at /var/lib/incus/logs/ with the pre-existing data moved there will not cause new instances from logging at the new location.

Also another small wrinkle, running instances will not like their existing log files being moved across mounts, that will effectively break logging for those. So we should add a check so the config key can only be changed if the system has no running instances.