qemu: Partially convert to net model enum

qemu: Partially convert to net model enum

This converts the qemu driver to the net model enum, for all
the model values that we have hardcoded for various checks,
which adds e1000e, virtio-transitional, virtio-non-transitional,
usb-net, spapr-vlan, lan9118, smc91c111

Because the qemu driver has historically also allowed the raw
model string onto the qemu command line, this isn't a full
conversion. Unwinding that will require more thought. However
for all new driver code we should be adding explicit enum
values for any model name we have special handling for.

Remove the now unused virDomainNetStreqModelString

Acked-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>