iommu/vt-d: Avoid format string leaks into iommu_device_create

This makes sure it won't be possible to accidentally leak format
strings into iommu device names. Current name allocations are safe,
but this makes the "%s" explicit.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>