kvm tools: Export DISPLAY ENV as our default host ip address

kvm tools: Export DISPLAY ENV as our default host ip address

It is useful to run a X program in guest and display it on host.

1) Make host's x server listen to localhost:6000
   host_shell$ socat -d -d TCP-LISTEN:6000,fork,bind=localhost \
               UNIX-CONNECT:/tmp/.X11-unix/X0

2) Start the guest and run X program
   host_shell$ lkvm run -k /boot/bzImage
  guest_shell$ xlogo

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>