qemu: vcpu: Clear vcpu order information rather than making it invalid

qemu: vcpu: Clear vcpu order information rather than making it invalid

Certain operations may make the vcpu order information invalid. Since
the order is primarily used to ensure migration compatibility and has
basically no other user benefits, clear the order prior to certain
operations and document that it may be cleared.

All the operations that would clear the order can still be properly
executed by defining a new domain configuration rather than using the
helper APIs.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1370357