vcpupin: introduce the new libvirt API (virDomainGetVcpupinInfo)

vcpupin: introduce the new libvirt API (virDomainGetVcpupinInfo)

This patch introduces a new libvirt API (virDomainGetVcpupinInfo),
as a counterpart to virDomainPinVcpuFlags.

We can use virDomainGetVcpus API to retrieve CPU affinity information,
but can't use this API against inactive domains (at least in case of KVM),
as it lacks a flags parameter.
The usual thing is to add a new virDomainGetVcpusFlags, but that API name
is already occupied by the counterpart to virDomainGetMaxVcpus, which
has a completely different signature.

The virDomainGetVcpupinInfo is the new API to retrieve CPU affinity
information of active and inactive domains.  While the usual convention
is to list an array before its length, this API violates that rule
in order to be more like virDomainGetVcpus (where maxinfo was doing
double-duty as the length of two different arrays).

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