introducing a DisplayAllocator interface

introducing a DisplayAllocator interface

This patch introduces a new DisplayAllocator interface to allow
frontends to allocate the DisplaySurface for the graphic device to use.
At the moment it is used only by xenfbfront, to be able to page align
the displaysurface and also keep the same backing data across multiple
DisplaySurface resize.
Howeveri in the near future it is going to be a useful improvement for
SDL as well.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>