[rfc] suggested offset properties + qxl support

There was some discussion on spice irc channel somewhere last week about how best to take note of the x/y offsets that were suggested by the hosts into the guests, so that the guest can configured the monitors in the same layout as they are on the host side.

I think initially the kernel needs to just propogate the values to userspace, so I've created two properties. The X server and desktop environments should start to use these properties to set things correctly.

Dave.