Commit 3f71c79768 added 'qemu_id' field to track the id of the cpu
as reported by query-cpus. The patch did not include changes necessary
to propagate the id through the functions matching the data to the
libvirt cpu structures and thus all vcpus had id 0.
The field is named 'enable_id' in other structures and a patch recently
added 'qemu_id' which has different semantics. To avoid confusion in the
tests rename the field.
Return whether a vcpu entry is hotpluggable or online so that upper
layers don't have to infer the information from other data.
Advantage is that this code can be tested by unit tests.
