virsh_vcpupin: use libvirt submodule for call

Commit id 'b94ccb677' missed the libvirt submodule reorganiation
for the 'cpu_allowed_list_by_task()' call resulting in the error:

AttributeError: 'module' object has no attribute 'cpu_allowed_list_by_task'
Merge pull request #1006 from jferlan/virsh_vcpupin

virsh_vcpupin: use libvirt submodule for call