implement map_volume and unmap_volume

This allows to request a mapped device/path explicitly, regardles of
the storage option, eg. krbd option in the RBDplugin.

Bump of the storage ABI => 2

Co-authored-by: Alwin Antreich <a.antreich@proxmox.com>
Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>

PVE/Storage/Plugin.pm

@@ -949,6 +949,18 @@ sub deactivate_storage {
     # do nothing by default
 }
 
+sub map_volume {
+    my ($class, $storeid, $scfg, $volname, $snapname) = @_;
+
+    return undef;
+}
+
+sub unmap_volume {
+    my ($class, $storeid, $scfg, $volname, $snapname) = @_;
+
+    return 1;
+}
+
 sub activate_volume {
     my ($class, $storeid, $scfg, $volname, $snapname, $cache) = @_;