fix space calculation for small disks for pve product

fix space calculation for small disks for pve product

The convoluted calculation logic in case the disks is 8GB leads to
datasize becoming 16EiB further down:
* after calculating and removing the rootsize from $rest, $rest becomes
  smaller than $space (which should be the minimal non-used space in the
  volume-group) - this leads to a negative value, which overflows in
  the `& ~0xFFF` opration.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>