ui/spice-display.c: add missing initialization for valgrind

We can't initialize QXLDevSurfaceCreate field by field because it has a
pa hole, and so 4 bytes remain uninitialized when building on x86-64, so
just memset.

Signed-off-by: Alon Levy <alevy@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
