qemu: Properly check return value of VIR_STRDUP in qemuDomainGetBlockIoTune

Setting the 'group_name' for a disk would falsely trigger a error path
as in commit 4b57f765 we did not properly check the return value of
VIR_STRDUP.

src/qemu/qemu_driver.c

@@ -17748,7 +17748,7 @@ qemuDomainGetBlockIoTune(virDomainPtr dom,
 
         /* Group name needs to be copied since qemuMonitorGetBlockIoThrottle
          * allocates it as well */
-        if (VIR_STRDUP(reply.group_name, disk->blkdeviotune.group_name))
+        if (VIR_STRDUP(reply.group_name, disk->blkdeviotune.group_name) < 0)
             goto endjob;
     }