Add fast path to qemu_spice_display_switch in case old and new
displaysurface have identical size (happens with display panning
and page flipping).  We just swap the backing store then and don't
go through the whole process of deleting and creating the primary
surface.

To simplify the code a bit move mirror surface allocation to
qemu_spice_display_switch().
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>