xen: import xenvirt pmd and vhost_xen

xen: import xenvirt pmd and vhost_xen

This provides a para-virtualization packet switching solution, based on the
Xen hypervisor’s Grant Table, which provides simple and fast packet
switching capability between guest domains and host domain based on
MAC address or VLAN tag.

This solution is comprised of two components; a Poll Mode Driver (PMD)
as the front end in the guest domain and a switching back end in the
host domain.  XenStore is used to exchange configure information
between the PMD front end and switching back end,
including grant reference IDs for shared Virtio RX/TX rings, MAC
address, device state, and so on.

The front end PMD can be found in the Intel DPDK directory lib/
librte_pmd_xenvirt and back end example in examples/vhost_xen.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>