dib: add tgz output format

Implement the "tgz" output format, i.e. "tar" compressed as gzip.

This was implemented in diskimage-builder upstream as
commit da41ee6012b064aa6901c871a1104a3a3933117e.

dib/Makefile.am

@@ -32,6 +32,7 @@ formats = \
 	qcow2 \
 	raw \
 	tar \
+	tgz \
 	vhd
 
 SOURCES_ML = \

dib/output_format_tgz.ml

@@ -0,0 +1,35 @@
+(* virt-dib
+ * Copyright (C) 2017 Red Hat Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ *)
+
+open Common_utils
+open Common_gettext.Gettext
+
+open Output_format
+
+let tgz_run_fs (g : Guestfs.guestfs) filename _ =
+  message (f_"Compressing the image as tar.gz");
+  g#tar_out ~excludes:[| "./sys/*"; "./proc/*" |] ~compress:"gzip"
+    "/" filename
+
+let fmt = {
+  defaults with
+    name = "tgz";
+    run_on_filesystem = Some tgz_run_fs;
+}
+
+let () = register_format fmt

dib/virt-dib.pod

@@ -216,6 +216,10 @@ Raw disk format.
 
 An uncompressed tarball.
 
+=item C<tgz>
+
+A tarball compressed with gzip.
+
 =item C<vhd>
 
 C<Virtual Hard Disk> disk image.  This output format requires