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.