When coldplugging vcpus to a VM that already has a few hotpluggable
vcpus the code might generate invalid configuration as
non-hotpluggable cpus need to be clustered starting from vcpu 0.

This fix forces the added vcpus to be hotpluggable in such case.

Fixes a corner case described in:
https://bugzilla.redhat.com/show_bug.cgi?id=1370357