virtio-gpu: fix information leak in capset get dispatch

In virgl_cmd_get_capset function, it uses g_malloc to allocate
a response struct to the guest. As the 'resp'struct hasn't been full
initialized it will lead the 'resp->padding' field to the guest.
Use g_malloc0 to avoid this.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 58188cae.4a6ec20a.3d2d1.aff2@mx.google.com

[ kraxel: resolved conflict ]

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