Improve Nova support for multiqueue network adapters

This spec proposes to update the libvirt driver with the configuration options for multiqueue operations and to add Packed Ring configuration option support.

At the moment, OpenStack has very limited support for multiqueue network adapters. Current implementation of multiqueue vhost(user) netdev defaults to the number of queues equal to the number of vCPUs. Such operations model does not allign well with the hardware-offloaded netdev usage model, where number of queues available in the underlying hardware is fixed. Ideally, OpenStack should allow the operator to start a VM with arbitrary number of vCPUs that is larger (or equal) to the number of queues configured on vNICs.