qemu_saveimage: add zstd to supported compression formats

Extend the list of supported formats, update and clarify comment
in qemu.conf.in (removed misleading sentence about the order of
compression format types).

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/589
Signed-off-by: Adam Julis <ajulis@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

libvirt.spec.in

@@ -815,6 +815,7 @@ Requires: gzip
 Requires: bzip2
 Requires: lzop
 Requires: xz
+Requires: zstd
 Requires: systemd-container
 Requires: swtpm-tools
         %if %{with_numad}

src/qemu/qemu.conf.in

@@ -582,10 +582,9 @@
 # memory from the domain is dumped out directly to a file.  If you have
 # guests with a large amount of memory, however, this can take up quite
 # a bit of space.  If you would like to compress the images while they
-# are being saved to disk, you can also set "lzop", "gzip", "bzip2", or "xz"
-# for save_image_format.  Note that this means you slow down the process of
-# saving a domain in order to save disk space; the list above is in descending
-# order by performance and ascending order by compression ratio.
+# are being saved to disk, you can also set "zstd", "lzop", "gzip", "bzip2",
+# or "xz" for save_image_format. Note that this means you slow down the process
+# of saving a domain in order to save disk space.
 #
 # save_image_format is used when you use 'virsh save' or 'virsh managedsave'
 # at scheduled saving, and it is an error if the specified save_image_format

src/qemu/qemu_saveimage.c

@@ -47,6 +47,7 @@ typedef enum {
      */
     QEMU_SAVE_FORMAT_XZ = 3,
     QEMU_SAVE_FORMAT_LZOP = 4,
+    QEMU_SAVE_FORMAT_ZSTD = 5,
     /* Note: add new members only at the end.
        These values are used in the on-disk format.
        Do not change or re-use numbers. */
@@ -62,6 +63,7 @@ VIR_ENUM_IMPL(qemuSaveCompression,
               "bzip2",
               "xz",
               "lzop",
+              "zstd",
 );
 
 static inline void