list volumes: also return backup type for backups

Otherwise, there is no storage-agnostic way to filter by backup group.

Call it subtype, to not confuse it with content type, and to be able
to re-use it for other content types than backup, if the need ever
arises.

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

PVE/Storage/PBSPlugin.pm

@@ -688,6 +688,7 @@ sub list_volumes {
 	    content => 'backup',
 	    vmid => int($bid),
 	    ctime => $epoch,
+	    subtype => $btype eq 'vm' ? 'qemu' : 'lxc', # convert to PVE backup type
 	};
 
 	$info->{verification} = $item->{verification} if defined($item->{verification});