vcpupin: add query option to virsh vcpupin command

vcpupin: add query option to virsh vcpupin command

This patch teaches "virsh vcpupin" command to query if no list
is given. Its feature is to show CPU affinity information in more
reader-friendly way.

 # virsh vcpupin VM --config
 VCPU: CPU Affinity
 ----------------------------------
    0: 1-6,9-20
    1: 10
    2: 5,9-11,15-20
    3: 1,3,5,7,9,11,13,15

When cpulist is omitted, vcpu number is optional. When vcpu number is
provided, information of only specified vcpu is displayed.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: Eric Blake <eblake@redhat.com>