virtio-balloon: Fix balloon not working correctly when hotplug memory

When do memory balloon, it takes the 'ram_size' as the VM's current ram size,
But 'ram_size' is the startup configured ram size, it does not take into
account the hotplugged memory.

As a result, the balloon result will be confused.
Steps to reproduce:
(1)Start VM: qemu -m size=1024,slots=4,maxmem=8G
(2)In VM: #free -m : 1024M
(3)qmp balloon 512M
(4)In VM: #free -m : 512M
(5)hotplug pc-dimm 1G
(6)In VM: #free -m : 1512M
(7)qmp balloon 256M
(8)In VM: #free -m :1256M
We expect the VM's available ram size to be 256M after 'qmp balloon 256M'
command, but VM's real available ram size is 1256M.

For "qmp balloon" is not performance critical code, we use function
'get_current_ram_size' to get VM's current ram size.
Signed-off-by: Nzhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

hw/virtio/virtio-balloon.c

@@ -294,10 +294,12 @@ static void virtio_balloon_set_config(VirtIODevice *vdev,
     VirtIOBalloon *dev = VIRTIO_BALLOON(vdev);
     struct virtio_balloon_config config;
     uint32_t oldactual = dev->actual;
+    ram_addr_t vm_ram_size = get_current_ram_size();
+
     memcpy(&config, config_data, sizeof(struct virtio_balloon_config));
     dev->actual = le32_to_cpu(config.actual);
     if (dev->actual != oldactual) {
-        qapi_event_send_balloon_change(ram_size -
+        qapi_event_send_balloon_change(vm_ram_size -
                         ((ram_addr_t) dev->actual << VIRTIO_BALLOON_PFN_SHIFT),
                         &error_abort);
     }
@@ -312,20 +314,21 @@ static uint32_t virtio_balloon_get_features(VirtIODevice *vdev, uint32_t f)
 static void virtio_balloon_stat(void *opaque, BalloonInfo *info)
 {
     VirtIOBalloon *dev = opaque;
-    info->actual = ram_size - ((uint64_t) dev->actual <<
-                               VIRTIO_BALLOON_PFN_SHIFT);
+    info->actual = get_current_ram_size() - ((uint64_t) dev->actual <<
+                                             VIRTIO_BALLOON_PFN_SHIFT);
 }
 
 static void virtio_balloon_to_target(void *opaque, ram_addr_t target)
 {
     VirtIOBalloon *dev = VIRTIO_BALLOON(opaque);
     VirtIODevice *vdev = VIRTIO_DEVICE(dev);
+    ram_addr_t vm_ram_size = get_current_ram_size();
 
-    if (target > ram_size) {
-        target = ram_size;
+    if (target > vm_ram_size) {
+        target = vm_ram_size;
     }
     if (target) {
-        dev->num_pages = (ram_size - target) >> VIRTIO_BALLOON_PFN_SHIFT;
+        dev->num_pages = (vm_ram_size - target) >> VIRTIO_BALLOON_PFN_SHIFT;
         virtio_notify_config(vdev);
     }
 }