lvmplugin: add external-snapshots option && forbid creation of qcow2 volumes without it

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

src/PVE/Storage/LVMPlugin.pm

@@ -399,6 +399,7 @@ sub options {
         base => { fixed => 1, optional => 1 },
         tagged_only => { optional => 1 },
         bwlimit => { optional => 1 },
+        'external-snapshots' => { optional => 1 },
     };
 }
 
@@ -603,6 +604,10 @@ my sub alloc_lvm_image {
 
     die "unsupported format '$fmt'" if $fmt ne 'raw' && $fmt ne 'qcow2';
 
+    die "external-snapshots option need to be enabled to use qcow2 format"
+        if $fmt eq 'qcow2'
+        && !$scfg->{'external-snapshots'};
+
     $class->parse_volname($name);
 
     my $vgs = lvm_vgs();