Whitespace: Neaten preliminary vGPU Xenops support

$ camlp4o /.../pa_type_conv.cma /.../pa_rpc.cma -printer \
  o -no_comments ocaml/xenops/xenops_server_xen.ml | md5sum
5b36d018813af9de49973dc5b19bfde9  -

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-5596: Move VGPU startup logic to Device module

VGPU emulator now gets started alongside Qemu since it's really part of the
emulation model.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-5596: Have VM_start fail if vgpu fails to start

Factored out the code to start Qemu into a function that forks a daemon and
waits for it to finish initialising (with a write to XenStore). This has then
be reused to kick off the vgpu emulator binary which can now fail in the same
way if it doesn't initialise. vgpu is kicked off in conjuction with Qemu since
it's part of the device model.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #1394 from simonjbeaumont/cp-5596

Start up vGPU emulation with Qemu