qcow2: add external snapshot support

add a snapext option to enable the feature

When a snapshot is taken, the current volume is renamed to snap volname
and a current image is created with the snap volume as backing file

Signed-off-by: Alexandre Derumier <alexandre.derumier@groupe-cyllene.com>

src/PVE/Storage/NFSPlugin.pm

@@ -104,6 +104,7 @@ sub options {
         'create-subdirs' => { optional => 1 },
         bwlimit => { optional => 1 },
         preallocation => { optional => 1 },
+        'external-snapshots' => { optional => 1, fixed => 1 },
     };
 }