hw/display/qxl: fix TOCTOU in cursor chunk data_size handling

Snapshot chunk.data_size into a host-local variable before passing it to
qxl_phys2virt() for validation, and pass it through qxl_cursor() and
qxl_unpack_chunks() so that no subsequent code re-reads the field.

Without this, a racing vCPU can inflate data_size between the
qxl_phys2virt() validation and the memcpy in qxl_unpack_chunks(),
causing a source read past the validated region. In practice the read
stays within the guest's own VRAM mmap, so the impact is limited.

Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3757
Reported-by: Feifan Qian <bea1e@proton.me>
Signed-off-by: Marc-Andre Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>
Message-ID: <20260710134352.2313675-1-marcandre.lureau@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@oss.qualcomm.com>