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

virtio-pci-net seems to have issues with more than three (3)
interfaces attached to the same guest, even on separate PCI buses.

To bypass this limitation, for now, we will use virtio-mmio for
PXE, and virtio-pci-net for the rest.

This keeps the guest eth order consistent and predictable, and
allows udev to assign predictable interface names for 3 out of 4
guest network interfaces (enp2sX), thus ensuring PXE will always
get the "eth0" designator.

Change-Id: I0d0cb8148cd87313e67f8f27793eb2414c1e90e3
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>