qemu: Don't overwrite errors by closefd in error paths

When qemuMonitorCloseFileHandle is called in error path, we need to
preserve the original error since a possible further error when running
closefd monitor command is not very useful to users.

src/qemu/qemu_monitor.c

@@ -1964,20 +1964,30 @@ int qemuMonitorSendFileHandle(qemuMonitorPtr mon,
 int qemuMonitorCloseFileHandle(qemuMonitorPtr mon,
                                const char *fdname)
 {
-    int ret;
+    int ret = -1;
+    virErrorPtr error;
+
     VIR_DEBUG("mon=%p fdname=%s",
           mon, fdname);
 
+    error = virSaveLastError();
+
     if (!mon) {
         qemuReportError(VIR_ERR_INVALID_ARG, "%s",
                         _("monitor must not be NULL"));
-        return -1;
+        goto cleanup;
     }
 
     if (mon->json)
         ret = qemuMonitorJSONCloseFileHandle(mon, fdname);
     else
         ret = qemuMonitorTextCloseFileHandle(mon, fdname);
+
+cleanup:
+    if (error) {
+        virSetError(error);
+        virFreeError(error);
+    }
     return ret;
 }
 

src/qemu/qemu_monitor.h

@@ -362,6 +362,9 @@ int qemuMonitorSendFileHandle(qemuMonitorPtr mon,
                               const char *fdname,
                               int fd);
 
+/* The function preserves previous error and only sets it's own error if no
+ * error was set before.
+ */
 int qemuMonitorCloseFileHandle(qemuMonitorPtr mon,
                                const char *fdname);