add check for fsfreeze before snapshot

In order to take a snapshot of a container volume, which can be mounted
read-only with RBD, the volume needs to be frozen (fsfreeze (8)) before taking
the snapshot.

This commit adds helpers to determine if the FIFREEZE ioctl needs to be called
for the volume.

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>

PVE/Storage/Plugin.pm

@@ -888,6 +888,10 @@ sub volume_snapshot_delete {
     return undef;
 }
 
+sub volume_snapshot_needs_fsfreeze {
+
+    return 0;
+}
 sub storage_can_replicate {
     my ($class, $scfg, $storeid, $format) = @_;