mount-image-callback: must specify format to qemu-nbd

at some point recently, qemu-nbd (and qemu also) started complaining about raw devices.

qemu-nbd will actually refuse to connect and exit failure unless you specify --format if it does not detect the format.

the end result is that on wily right now:
  mount-image-callback partititioned-disk-image.raw /bin/bash

will fail.

Additionally, because of the way it fails, it actually never disconnects from the device.

It looks like this:

$ sudo mount-image-callback -vvv /tmp/disk.img -- /bin/bash
WARNING: Image format was not specified for '/tmp/disk.img' and probing guessed raw.
         Automatically detecting the format is dangerous for raw images, write operations on block 0 will be restricted.
         Specify the 'raw' format explicitly to remove the restrictions.
failed to qemu-nbd connect /tmp/disk.img to /dev/nbd0

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: cloud-image-utils 0.27-0ubuntu18
ProcVersionSignature: Ubuntu 4.1.0-3.3-generic 4.1.3
Uname: Linux 4.1.0-3-generic x86_64
ApportVersion: 2.18-0ubuntu9
Architecture: amd64
CurrentDesktop: Unity
Date: Fri Sep 4 12:32:24 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-07-23 (43 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
PackageArchitecture: all
SourcePackage: cloud-utils
UpgradeStatus: No upgrade log present (probably fresh install)