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}