[libvirt] PATCH: allow to set nvcpus = 0

OpenVZ uses all CPUs available in system - by default (number of CPUs did not set) - number of CPUs = 0 Currenty, libvirt don't allow to set nvcpus = 0. Attached patch removes limitation in libvirt set nvcpu = 0, but add it to each driver which allow to set number of virtual CPU. For OpenVZ set default number of CPUs = 0.