revert default compress mode for vzdump to 0

revert default compress mode for vzdump to 0

rationale is the following:

we had this as default before commit
1ab98f059e1d28e5dd28f326ab371d6d5c1b8bb4

but wrongly documented

since we set the compression mode explicitly via the gui,
we can set the default to 0, which makes some operations more
intuitive (e.g. with the -stdout parameter it makes no sense
to compress it by default)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>