Instead of trying to figure out what the proper ImageModel is for
the instance's image, just use the get_model function. The resulting
ImageModel will actually represent the proper ImageModel for the
instance, and downstream mounting functions will be able to make
intelligent decisions on how to mount the instance's disks.
It is not possible to use get_model for an instance booted from volume,
but we can still use LocalBlockImage in it's case because it is
simply exposed as a block device.
Partial-Bug: #1287754
Change-Id: I0f4456731c9d6f64affbf6924f4a7a4c34df4fd1
faeae32a7e