The qemuDomainHelperGetVcpus attempted to report an error when the
vcpupids info was NULL. Unfortunately earlier code would clamp the
value of 'maxinfo' to 0 when nvcpupids was 0, so the error reporting
would end up being skipped.

This lead to 'virsh vcpuinfo <dom>' just returning an empty list
instead of giving the user a clear error.