lxc: Don't accidentaly reset autostart flag in virLXCProcessCleanup
virDomainDeleteConfig is meant to delete the persistent config and thus
it resets vm->autostart. Copy parts of qemuProcessRemoveDomainStatus to
a new helper to avoid using the incorrect function.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1230071