config: rename external-snapshots to snapshot-as-volume-chain

Not perfect but now it's still easy to rename and the new variant fits
a bit better to the actual design and implementation.

Add best-effort migration for storage.cfg, this has been never
publicly released after all.

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

src/PVE/API2/Storage/Status.pm

@@ -223,7 +223,7 @@ __PACKAGE__->register_method({
             # TODO: add support to the storage plugin system to allow returing different supported
             # formats depending on the storage config instead, this is just a stop gap!
             if (lc($data->{type}) eq 'lvm') {
-                $data->{format}[0]->{qcow2} = 0 if !$scfg->{'external-snapshots'};
+                $data->{format}->[0]->{qcow2} = 0 if !$scfg->{'snapshot-as-volume-chain'};
             }
 
             $res->{$storeid} = $data;