Virsh.vcpupin: Let 'vcpu' and 'cpu_list' optional

Omit 'vcpu_list' to query given vcpu affinity, and omit both 'vcpu'
and 'cpu_list' to query all domain vcpu affinity.

Signed-off-by: Yanbing Du <ydu@redhat.com>
Merge pull request #1773 from will-Do/cpupin

Virsh.vcpupin: Let 'vcpu' and 'cpu_list' optional