Pinning to all physical cpus means resetting, hence it is preferable to
delete vcpupin setting of XML.

This patch changes qemu driver to delete vcpupin setting by invoking
virDomainVcpupinDel API when pinning the specified virtual cpu to
all host physical cpus.
Signed-off-by: NTaku Izumi <izumi.taku@jp.fujitsu.com>