virt: move get_cpuset_ids into nova.virt.hardware

The nova.virt.cpu module has a single helper method for
getting the instance vCPU pin list. The new module
nova.virt.hardware is intended to be a single place for
all helpers related to guest hardware configuration, so
it is a natural place for the get_cpuset_ids method.
Rename it to 'get_vcpu_pin_set' to slightly clarify its
purpose too.

Blueprint: virt-driver-numa-placement
Blueprint: virt-driver-cpu-pinning
Change-Id: Id106737dc40471e8d31fbc002256b70e0b6ad5f6
Merge "virt: move get_cpuset_ids into nova.virt.hardware"