libxl: add basic spice support for pv domUs

This patch adds basic spice support for pv domUs, qemu parameters
are the same as hvm ones and working.
Therefore xl cfg paramters are the same as 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.
Any feedback is appreciated.

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