qemu: Use a TAP device instead of slirp for networking
With this patch, a persistent TAP device is set up by poky-qemu-ifup,
which is now run before qemu. The qemu command line now uses the
device that was constructed (rather than the hard-coded tap0) and it
is told not to run any networking scripts.
When qemu shuts down, poky-qemu-ifdown removes the TAP device.
sudo use - sudo is used to run poky-qemu-ifup. sudo is no longer used
to run qemu, as qemu no longer needs privileges to set up networking.
poky-qemu-ifdown is run without privileges, as you can remove a TAP
device which you own.
