virsh: Add 'echo --err' option

Since test:///default resets state on every connection, writing a test
that covers a sequence of commands must be done from a single
session. But if the test wants to exercise particular failure modes as
well as successes, it can be nice to leave witnesses in the stderr
stream immediately before and after the spot where the expected error
should be, to ensure the rest of the script is not causing errors.

Do this by adding an --err option.
Signed-off-by: NEric Blake <eblake@redhat.com>
Acked-by: NMichal Privoznik <mprivozn@redhat.com>

tools/virsh.pod

@@ -623,12 +623,14 @@ the usable CPU models are only limited by the hypervisor. This command will
 print that all CPU models are accepted for these architectures and the actual
 list of supported CPU models can be checked in the domain capabilities XML.
 
-=item B<echo> [I<--shell>] [I<--xml>] [I<arg>...]
+=item B<echo> [I<--shell>] [I<--xml>] [I<err>...] [I<arg>...]
 
 Echo back each I<arg>, separated by space.  If I<--shell> is
 specified, then the output will be single-quoted where needed, so that
 it is suitable for reuse in a shell context.  If I<--xml> is
 specified, then the output will be escaped for use in XML.
+If I<--err> is specified, prefix B<"error: "> and output to stderr
+instead of stdout.
 
 =item B<hypervisor-cpu-compare> I<FILE> [I<virttype>] [I<emulator>] [I<arch>]
 [I<machine>] [I<--error>]

tools/vsh.c

@@ -3296,6 +3296,10 @@ const vshCmdOptDef opts_echo[] = {
      .type = VSH_OT_BOOL,
      .help = N_("escape for XML use")
     },
+    {.name = "err",
+     .type = VSH_OT_BOOL,
+     .help = N_("output to stderr"),
+    },
     {.name = "str",
      .type = VSH_OT_ALIAS,
      .help = "string"
@@ -3329,6 +3333,7 @@ cmdEcho(vshControl *ctl, const vshCmd *cmd)
 {
     bool shell = false;
     bool xml = false;
+    bool err = false;
     int count = 0;
     const vshCmdOpt *opt = NULL;
     char *arg;
@@ -3338,6 +3343,8 @@ cmdEcho(vshControl *ctl, const vshCmd *cmd)
         shell = true;
     if (vshCommandOptBool(cmd, "xml"))
         xml = true;
+    if (vshCommandOptBool(cmd, "err"))
+        err = true;
 
     while ((opt = vshCommandOptArgv(ctl, cmd, opt))) {
         char *str;
@@ -3372,8 +3379,12 @@ cmdEcho(vshControl *ctl, const vshCmd *cmd)
         return false;
     }
     arg = virBufferContentAndReset(&buf);
-    if (arg)
-        vshPrint(ctl, "%s", arg);
+    if (arg) {
+        if (err)
+            vshError(ctl, "%s", arg);
+        else
+            vshPrint(ctl, "%s", arg);
+    }
     VIR_FREE(arg);
     return true;
 }