qemu: cleanup error checking on agent replies

On all the places where qemuAgentComand() was called, we did a check
for errors in the reply.  Unfortunately, some of the places called
qemuAgentCheckError() without checking for non-null reply which might
have resulted in a crash.

So this patch makes the error-checking part of qemuAgentCommand()
itself, which:

 a) makes it look better,

 b) makes the check mandatory and, most importantly,

 c) checks for the errors if and only if it is appropriate.

This actually fixes a potential crashers when qemuAgentComand()
returned 0, but reply was NULL.  Having said that, it *should* fix the
following bug:

https://bugzilla.redhat.com/show_bug.cgi?id=1058149Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

src/qemu/qemu_agent.c

@@ -117,6 +117,8 @@ struct _qemuAgent {
     qemuAgentEvent await_event;
 };
 
+static int qemuAgentCheckError(virJSONValuePtr cmd, virJSONValuePtr reply);
+
 static virClassPtr qemuAgentClass;
 static void qemuAgentDispose(void *obj);
 
@@ -1009,6 +1011,7 @@ qemuAgentCommand(qemuAgentPtr mon,
             }
         } else {
             *reply = msg.rxObject;
+            ret = qemuAgentCheckError(cmd, *reply);
         }
     }
 
@@ -1277,9 +1280,6 @@ int qemuAgentShutdown(qemuAgentPtr mon,
     ret = qemuAgentCommand(mon, cmd, &reply,
                            VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK);
 
-    if (reply && ret == 0)
-        ret = qemuAgentCheckError(cmd, reply);
-
     virJSONValueFree(cmd);
     virJSONValueFree(reply);
     return ret;
@@ -1308,8 +1308,7 @@ int qemuAgentFSFreeze(qemuAgentPtr mon)
         return -1;
 
     if (qemuAgentCommand(mon, cmd, &reply,
-                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0 ||
-        qemuAgentCheckError(cmd, reply) < 0)
+                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0)
         goto cleanup;
 
     if (virJSONValueObjectGetNumberInt(reply, "return", &ret) < 0) {
@@ -1346,8 +1345,7 @@ int qemuAgentFSThaw(qemuAgentPtr mon)
         return -1;
 
     if (qemuAgentCommand(mon, cmd, &reply,
-                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0 ||
-        qemuAgentCheckError(cmd, reply) < 0)
+                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0)
         goto cleanup;
 
     if (virJSONValueObjectGetNumberInt(reply, "return", &ret) < 0) {
@@ -1386,9 +1384,6 @@ qemuAgentSuspend(qemuAgentPtr mon,
     ret = qemuAgentCommand(mon, cmd, &reply,
                            VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK);
 
-    if (reply && ret == 0)
-        ret = qemuAgentCheckError(cmd, reply);
-
     virJSONValueFree(cmd);
     virJSONValueFree(reply);
     return ret;
@@ -1419,9 +1414,6 @@ qemuAgentArbitraryCommand(qemuAgentPtr mon,
     if ((ret = qemuAgentCommand(mon, cmd, &reply, timeout)) < 0)
         goto cleanup;
 
-    if ((ret = qemuAgentCheckError(cmd, reply)) < 0)
-        goto cleanup;
-
     if (!(*result = virJSONValueToString(reply, false)))
         ret = -1;
 
@@ -1449,9 +1441,6 @@ qemuAgentFSTrim(qemuAgentPtr mon,
     ret = qemuAgentCommand(mon, cmd, &reply,
                            VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK);
 
-    if (reply && ret == 0)
-        ret = qemuAgentCheckError(cmd, reply);
-
     virJSONValueFree(cmd);
     virJSONValueFree(reply);
     return ret;
@@ -1472,8 +1461,7 @@ qemuAgentGetVCPUs(qemuAgentPtr mon,
         return -1;
 
     if (qemuAgentCommand(mon, cmd, &reply,
-                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0 ||
-        qemuAgentCheckError(cmd, reply) < 0)
+                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0)
         goto cleanup;
 
     if (!(data = virJSONValueObjectGet(reply, "return"))) {
@@ -1581,8 +1569,7 @@ qemuAgentSetVCPUs(qemuAgentPtr mon,
     cpus = NULL;
 
     if (qemuAgentCommand(mon, cmd, &reply,
-                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0 ||
-        qemuAgentCheckError(cmd, reply) < 0)
+                         VIR_DOMAIN_QEMU_AGENT_COMMAND_BLOCK) < 0)
         goto cleanup;
 
     if (virJSONValueObjectGetNumberInt(reply, "return", &ret) < 0) {