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});

PVE/Storage/Plugin.pm

@@ -1164,6 +1164,7 @@ my $get_subdir_files = sub {
 	    my $archive_info = eval { PVE::Storage::archive_info($fn) } // {};
 
 	    $info->{ctime} = $archive_info->{ctime} if defined($archive_info->{ctime});
+	    $info->{subtype} = $archive_info->{type} // 'unknown';
 
 	    if (defined($vmid) || $fn =~ m!\-([1-9][0-9]{2,8})\-[^/]+\.${format}$!) {
 		$info->{vmid} = $vmid // $1;