xenapi: add support for vcpu_pin_set

This moves vcpu_pin_set config from the libvirt driver into a place that
can be shared between drivers, nova.virt.cpu

At the same time, the utility function that converts the configuration
value into a list of integers has also been moved into nova.virt.cpu

This new code is then used in the XenAPI driver to apply a CPU mask that
implements the currently configured value for vcpu_pin_set

The main use case for this setting is to ensure all guest VMs are not
using the physical CPUs that Dom0 are configured to use.

DocImpact
Completes blueprint xenapi-vcpu-pin-set
Change-Id: I9d53d93db0c5f2a11ad685d3fc335f0cd6b1f79b
Merge "xenapi: add support for vcpu_pin_set"