Add prune_backups to storage API

Implement it for generic storages supporting backups
(i.e. directory-based storages) and add a wrapper for PBS.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>

test/run_plugin_tests.pl

@@ -16,6 +16,7 @@ my $res = $harness->runtests(
     "path_to_volume_id_test.pm",
     "get_subdir_test.pm",
     "filesystem_path_test.pm",
+    "prune_backups_test.pm",
 );
 
 exit -1 if !$res || $res->{failed} || $res->{parse_errors};