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