always align rootdisk size to 4 MiB

always align rootdisk size to 4 MiB

While this was already done in the $rest < 48 GiB cases, it wasn't yet
done for the else branch and also not if $maxroot_mb was assigned,
because of being smaller.

Second and last step towards fixing an issue reported in the community
forum [0] where using 250.00 hdsize, 250 maxroot and 0 minfree would
fail.

Turns out two extents would be missing because of lvcreate implicitly
rounding up, one of them for the root LV (the one for metadata was
already handled in the previous commit).

[0]: https://forum.proxmox.com/threads/129320/post-566375

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>