xen: add net backend driver. (Gerd Hoffmann)

xen: add net backend driver. (Gerd Hoffmann)

This patch adds a network interface backend driver to qemu.  It is a pure
userspace implemention using the gntdev interface.  It uses "qnet" as
backend name in xenstore so it doesn't interfere with the netback
backend (aka "vnif").

The network backend is hooked into the corrosponding qemu vlan, i.e.
vif 0 is hooked into vlan 0.  To make the packages actually arrive
somewhere you additionally have to link the vlan to the outside world
using the usual qemu command line options such as "-net tap,...".

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@7224 c046a42c-6fe2-441c-8c8c-71466251a162