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>
I guess you mean this case:
```
+ - invalid_vcpu_count_is_5:
+       setvcpus_count = "5"
```
here, ```vmxml.set_vm_vcpus(vm_name, max_vcpu, current_vcpu)``` is preapre step, and ```current_vcpu``` is not for test(```setvcpu_count``` does the right job). I think you may confuse these two params