drm/i915/gvt: Use snprintf() to prevent possible buffer overflow.

For printing the intel_vgpu->id, a buffer with fixed length is allocated
on the stack. But if vgpu->id is greater than 6 characters, the buffer
overflow will happen. Even the string of the amount of max vgpu is less
that the length buffer right now, it's better to replace sprintf() with
snprintf().

v2:
- Increase the size of the buffer. (Colin Xu)

This patch fixed the critical issue #673 reported by klocwork.

Signed-off-by: Aleksei Gimbitskii <aleksei.gimbitskii@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Colin Xu <colin.xu@intel.com>
Reviewed-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>