kvmtool: Add minimal support for macvtap

In order to be useable by kvmtool, a macvtap interface requires
some minimal configuration (basically setting up the offload bits).
This requires skipping some of the low level TUN/TAP setup.

To avoid adding yet another option, we extend the 'tapif' option
to detect the use of a file (such as /dev/tap23).

Assuming you've run the following as root:

	# ip link add link eth0 name kvmtap0 type macvtap mode bridge
	# chgrp kvm /dev/tap$(< /sys/class/net/kvmtap0/ifindex)
	# chmod g+rw /dev/tap$(< /sys/class/net/kvmtap0/ifindex)

it is fairly easy to have a script that does the following:

	#!/bin/sh
	addr=$(< /sys/class/net/kvmtap0/address)
	tap=/dev/tap$(< /sys/class/net/kvmtap0/ifindex)

kvmtool/lkvm run --console virtio			\
	-k /boot/zImage					\
	-p "console=hvc0 earlyprintk"			\
	-n trans=mmio,mode=tap,tapif=$tap,guest_mac=$addr

and you now have your VM running, directly attached to the network.

This patch also removes the TUNSETNOCSUM ioctl that has declared
obsolete for quite some time now...

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>