setvcpus: Case update and add new cases

1. The original test hard code the maximum vcpu number to 2, and the
test steps are not clear and maintain, so re-organize the case to make
it more clear.
2. Add a case to test vcpu hotplug without <acpi> feature
3. Add a case to test maximum CPUs exceeding topology limit.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Oh, my bad, it's `setvcpus_count=5`, not `current_vcpu=5`, please ignore this comment.