vcpupin: add reset option to virsh vcpupin command

vcpupin: add reset option to virsh vcpupin command

When resetting vcpupin setting, we have to specify all host physical
cpus as a cpulist parameter of virsh vcpupin command. It's a little
tedious.

This patch changes to allow to receive the special keyword 'r' as a cpulist
parameter of virsh vcpupin command when resetting vcpupin setting.

If you set the following:

 # virsh vcpupin VM 0 r

the vcpu0 will be pinned to all physical cpus.

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