lvm create: rework auto-space distribution and improve for low size setups

lvm create: rework auto-space distribution and improve for low size setups

Avoid a per-default extra free space block if the disk has less than
32 GB of total space.

Use the full rest size for root if it's below 12 GB, not much of a
point in setting up a data thin LVM for guests then.

Then also show a display message if no thin LVM is created to inform
the admin

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>