image cache size

[pronix]

code and unit tests for issue cinder 60

[Scsabiii]

This is not exaclty what i wanted.
The volume size should match the image.virtual_size only before calling the
self._register_image_in_cache(volume, image_id)
Than we need to extend the volume to the volume.size:
image_utils.resize_image(self.local_path(volume), volume.size,
run_as_root=self._execute_as_root)
data = image_utils.qemu_img_info(self.local_path(volume),
run_as_root=self._execute_as_root)
virt_size = data.virtual_size // units.Gi
if virt_size != volume.size:
raise exception.ImageUnacceptable.....

Your patch restricts the volume.size == image.virtual_size.
What i want is only to make the cached image small, so the cache should still have image.virtual_size
And the volume.size can be anyting >= to the image.virtual_size....
So the matching is ok, but it can be bigger too...

[Scsabiii]

Look at:
https://gist.github.com/Scsabiii/7f0ef886dd422e880afc96148aac000a