vhost: add interface name for virtio

vhost: add interface name for virtio

This patch fixes the issue whereby when using userspace vhost ports
in the context of vSwitching, the name provided to the hypervisor/QEMU
of the vhost tap device needs to be exposed in the library, in order
for the vSwitch to be able to direct packets to the correct device.
This patch introduces an 'ifname' member to the virtio-net structure
which is populated with the tap device name when QEMU is brought up
with a vhost device.

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>
Signed-off-by: Anthony Fee <anthonyx.fee@intel.com>
Acked-by: Huawei Xie <huawei.xie@intel.com>