ioemu: cleaning DisplayState->dpy_resize interface

ioemu: cleaning DisplayState->dpy_resize interface

Bring the DisplayState dpy_resize interface back to how it is in qemu
mainstream, thus making the code easier to merge.
In order to support sharing the framebuffer, I am adding a new resize
interface called dpy_resize_shared that also has a depth and a pixels
parameters. As a consequence I could remove the dpy_colourdepth
callback and make the code cleaner and easier to read.

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