Create a 10 gibibytes diskimage instead of 10 gigabytes

Create a 10 gibibytes diskimage instead of 10 gigabytes

Otherwise the disk import may fail on block storages requiring
volumes size to be a multiple of the block size, like ZFS.

This fixes the error:
'volume size must be a multiple of volume block size'
when importing the generated disk image as a zvol.

Signed-off-by: Emmanuel Kasper <e.kasper@proxmox.com>