conf: Initialize the pinning policy for vcpus

conf: Initialize the pinning policy for vcpus

Document for <vcpu>'s "cpuset" says:

Since 0.4.4, this element can contain an optional cpuset attribute,
which is a comma-separated list of physical CPU numbers that virtual
CPUs can be pinned to.

However, it's not the truth, libvirt actually pins the domain
process to the specified pCPUs by "cpuset" of <vcpu>. And the
vcpu thread are pinned to all available pCPUs if no <vcpupin>
is specified for it.

This patch is to implement the codes to inherit <vcpu>'s "cpuset" for
vcpu that doesn't have <vcpupin> specified, and <vcpupin>
for these vcpu will be ignored when formating. Underlying
driver implementation will make sure the vcpu thread pinned
to correct pCPUs.