Previously, the size was rounded down which, in case of an image with
non-1KiB-aligned sze (only possible for external plugins or manually
created images) would lead to errors when attempting to write beyond
the end of the too small allocated target image.
For image allocation, the size is already rounded up to the
granularity of the storage. Do the same for import.
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>