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 is tested and working, except for the pointer not
visible but working.
Any feedback is appreciated.

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

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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.
libxl: add spice configuration values to xenstore

Add the acquisition of spice listen address and port from qmp
and store them.
Add spice basic configuration values written in xenstore when
libxl__device_vfb_add function is called.

Note: this patch is only a draft and needs to be improved.
Any feedback is appreciated.

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