The --image-size argument no longer correctly sizes the resulting image [2.0]

[Łukasz Zemczak]

It seems that --image-size doesn't quite work as expected. The end disk is not sized properly:

$ ubuntu-image snap -d -w test --image-size 8G -O ./output/dragonboard-18-beta ./models/ubuntu-core-18-amd64.model
$ fdisk -l pc.img
GPT PMBR size mismatch (1060947 != 1062995) will be corrected by write.
The backup GPT table is not on the end of the device.
Disk pc.img: 519.04 MiB, 544253952 bytes, 1062996 sectors

It can be seen that the end size of the disk is ~544MB instead of 8G. Comparing this to what we had in the previous ubuntu-image:

$ fdisk -l pc.img
Disk pc.img: 8 GiB, 8589934592 bytes, 16777216 sectors

(Note to self: we need to make sure that we have a new testcase for each of these bugs)