Clean up redundant usage of virDomainObjSetDefTransient

Clean up redundant usage of virDomainObjSetDefTransient

Commit 45ec297d from November 2010:
    Make state driver device hotplug/update actually transient
added virDomainObjSetDefTransient calls to the domain startup
function in several drivers.

In November 2011, commit 8866eed:
    Set aliases for LXC/UML console devices
added a call earlier in the startup function, without removing the
existing ones.

Also, in the UML driver it seems the function never did anything
useful - vm->def->id is set asynchronnously in umlNotifyEvent.
At the time of calling virDomainObjSetDefTransient with live=false,
vm->def->id was likely still -1, making the call a no-op.