When a user calls to virDomain{Attach,Detach,Update}DeviceFlags() with
flags == VIR_DOMAIN_DEVICE_MODIFY_LIVE on an inactive guest running on
an old Xen hypervisor (such as RHEL-5) xend_internal driver reports:

    Xend version does not support modifying persistent config

which is pretty confusing since no-one requested to modify persistent
config.