[v2] qemu: Improve docs for virsh dump format

[Osier Yang <jyang@xxxxxxxxxx>]

The error in getCompressionType will never be reported, change
the errors codes into warning (VIR_WARN("%s", _(foo)); doesn't break
syntax-check rule), and also improve the docs in qemu.conf to tell
user the truth.
---
 src/qemu/qemu.conf     |   10 ++++++++--
 src/qemu/qemu_driver.c |   15 ++++++++-------
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/src/qemu/qemu.conf b/src/qemu/qemu.conf
index 145062c..7de6863 100644
--- a/src/qemu/qemu.conf
+++ b/src/qemu/qemu.conf
@@ -196,8 +196,14 @@
 # saving a domain in order to save disk space; the list above is in descending
 # order by performance and ascending order by compression ratio.
 #
-# save_image_format is used when you use 'virsh save' at scheduled saving.
-# dump_image_format is used when you use 'virsh dump' at emergency crashdump.
+# save_image_format is used when you use 'virsh save' at scheduled
+# saving, and it is an error if the specified save_image_format is
+# not valid, or the according compression program can't be found.
+#
+# dump_image_format is used when you use 'virsh dump' at emergency
+# crashdump, and if the specified dump_image_format is not valid, or
+# the according compression program can't be found, this falls
+# back to "raw" compression.
 #
 # save_image_format = "raw"
 # dump_image_format = "raw"
diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index 0066c55..e9bd421 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -2712,17 +2712,18 @@ getCompressionType(struct qemud_driver *driver)
      */
     if (driver->dumpImageFormat) {
         compress = qemudSaveCompressionTypeFromString(driver->dumpImageFormat);
+        /* Use "raw" as the format if the specified format is not valid,
+         * or the compress program is not available,
+         */
         if (compress < 0) {
-            qemuReportError(VIR_ERR_OPERATION_FAILED, "%s",
-                            _("Invalid dump image format specified in "
-                              "configuration file, using raw"));
+            VIR_WARN("%s", _("Invalid dump image format specified in "
+                             "configuration file, using raw"));
             return QEMUD_SAVE_FORMAT_RAW;
         }
         if (!qemudCompressProgramAvailable(compress)) {
-            qemuReportError(VIR_ERR_OPERATION_FAILED,
-                            "%s", _("Compression program for dump image format "
-                                    "in configuration file isn't available, "
-                                    "using raw"));
+            VIR_WARN("%s", _("Compression program for dump image format "
+                             "in configuration file isn't available, "
+                             "using raw"));
             return QEMUD_SAVE_FORMAT_RAW;
         }
     }
-- 
1.7.6

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