vzdump: schema: add fleecing property string

vzdump: schema: add fleecing property string

It's a property string, because that avoids having an implicit
"enabled" as part of a 'fleecing-storage' property. And there likely
will be more options in the future, e.g. threshold/limit for the
fleecing image size.

Storage is non-optional, so the storage choice needs to be a conscious
decision. Can allow for a default later, when a good choice can be
made further down the stack. The original idea with "same storage as
VM disk" is not great, because e.g. for LVM, it would require the same
size as the disk up front.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
 [ TL: style fix for whitespace placement in multi-line strings ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>