kvmtool: convert net backend to support bi-endianness

Configure the queues to follow the guest endianness, and make sure
the configuration space is doing the same.

Extra care is taken for the handling of the virtio_net_hdr structures
on both the TX and RX ends.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>