ui/spice-display: use uintptr_t when casting qxl physical addresses

ui/spice-display: use uintptr_t when casting qxl physical addresses

The current intptr_t casts are a problem when the address's highest
bit is 1, and it is cast to a intptr_t and then to uint64_t, such
as at:
     surface.mem        = (intptr_t)ssd->buf;

This causes the sign bit to be extended which causes a wrong address to
be passed on to spice, which then complains when it gets the wrong
slot_id number, since the slot_id is taken from the higher bits.

The assertion happens early - during the first primary surface creation.

This fixes running "-vga qxl -spice" with 32 bit compiled
qemu-system-i386.

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