qemu: libxl: vcpupin: Don't reset pinning when pinning to all pcpus

qemu: libxl: vcpupin: Don't reset pinning when pinning to all pcpus

In the pre-NUMA ages pinning a vCPU to all pCPUs was eaqual to deleting
the pinning info. Now it does not entirely work that way. Pinning a vCPU
to all pCPUs might be a desired operation. Additionally removal of the
pinning will result into using the default pinning information at the
next boot which might be different from all vcpus.

This patch removes the false assumption that we should remove the
pinning after pinning to all vCPUs and tweaks the documentation for
virsh.

A later patch will implement a new flag for the virDomainPinVcpuFlags
API that will allow to remove the pinning in a sane way.