p/fuel: virtio: min libvirt 3.x and Qemu 2.6

p/fuel: virtio: min libvirt 3.x and Qemu 2.6

Expect host virtualization tools (libvirt, qemu) to be recent enough
to support modern virtio-net-pci out of box as a default.

This allows us to drop some changes and a hack - see [1]:
- "model=virtio" implies "model=virtio-net-pci" on libvirt 3.x;
- "model=virtio" puts each NIC on a seperate bus, all work now;
- modern virtio is automatically used when NIC is on PCIe bus;

Revert "p/fuel: lib.sh: Revert to virtio-mmio for PXE net"

This reverts commit 4f72e6be436badd2ded546424ec4a304cd6ce5d3.

[1] https://lists.gnu.org/archive/html/qemu-devel/2017-07/msg07594.html

Change-Id: Iaa7af63846c9d9dca5b7a9be5efa54f8cd56242e
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>