libxl: set DISPLAY and XAUTHORITY if sdl is enabled

libxl: set DISPLAY and XAUTHORITY if sdl is enabled

With this change the following domU.cfg will show the VM window on the
local dom0 display, instead of the remote side (it via ssh -X login):
  vnc=0
  sdl=1
  display=":0"
  xauthority="/run/gdm/auth-for-olaf-nMXhOi/database"

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>