drivers: ethernet: Add VIRTIO Network Device

Add a driver for the VIRTIO Ethernet device.
This is a minimal driver which sets a MAC address and transmits packets,
but does not support any extra features like the control channel or
checksum offloading.
Confirmed to work with the networking subsystem samples. For example, the
zperf sample shows a result of 85 Mbps download and 14 Mbps upload.

Signed-off-by: Jakub Klimczak <jklimczak@internships.antmicro.com>