Fix sheepdog volume creation from image

When creating a volume from an image, with sheepdog as the backend, cinder will try to run:

sudo cinder-rootwrap /etc/cinder/rootwrap.conf qemu-img convert -O raw <open GreenPipe '<fd:12>', mode 'w+b' at 0x35d6e60> sheepdog:volume-59e3326f-0f04-4205-8a78-a7669493d9bf

This is due to passing to image_utils.covert_image the tmp object, rather than the tmp.name