Quieten disk-image-get-kernel

disk-image-get-kernel is pretty noisy and you see e.g.:

  $> load-image overcloud-compute.qcow2
  XXX  -d '/tmp/image.lWGCgPoj' -o 'tmp' -i '/home/stack/overcloud-compute.qcow2' --
  Extracting kernel + ramdisk from /home/stack/overcloud-compute.qcow2 and writing them to /tmp/image.lWGCgPoj
  nbd                    17554  0
  nbd                    17554  0
  basename: missing operand
  Try 'basename --help' for more information.
  /dev/nbd0 disconnected
  tmp-vmlinuz,tmp-initrd

Clean all this up so we just get:

  $> load-image overcloud-compute.qcow2
  Extracting kernel + ramdisk from /home/stack/overcloud-compute.qcow2 to tmp-vmlinuz and tmp-initrd in /tmp/image.g6b0lG88

Change-Id: I8971ec0bbcd87157b07fc17254c56bb9f9f2a597
Merge "Quieten disk-image-get-kernel"