libxl: add basic spice support for pv domUs

This patch adds basic spice support for pv domUs.
The qemu parameters are the same as the hvm ones and they works.
Therefore xl cfg paramters are the same as the hvm ones except that
features not supported yet by pv domUs (vdagent and usbredirection)
are kept disabled by default.
It also enables vfb and vkb required to have basic spice working.

Note: this patch is only a draft and needs to be improved.
Patch v3 is tested and working, except for the pointer not
visible but working.
Patch v4 is not tested now and I now use the api for test
the u.hvm.spice retro-compatibility.
Any feedback is appreciated.

Signed-off-by: Fabio Fantoni <fabio.fantoni@m2r.biz>

---

Changes in v5:
- libxl__need_xenpv_qemu to true also if spice is enabled
- libxl_create.c: don't copy u.hvm.spice in the newer if
  the newer is already used
- xl_cmdimpl.c: parse all spice options out of hvm part
  and the features not supported will be disabled and will
  show a warning about them if was setted enabled

Changes in v4:
- added libxl.h changes
- libxl_create.c: added older u.hvm.spice compatibility
  copying it in newer one

Changes in v3:
- xl.cfg.pod.5: moved spice out of hvm section and specified
  the features for now hvm only.
- libxl_types.idl: added spice struct out of keyedunion hvm only.
- use new generic spice struct instead of hvm only ones.

Changes in v2:
- xl_cmdimpl.c: always set vnc and sdl toplevel parameters in &vfb
  with vnc or spice enabled on pv domUs otherwise in some cases it
  would fail with error for one bool default value missing.
- libxl_dm.c: do not add -nographic if spice is enabled, even though
  -nographic seems buggy in upstream qemu.