nvme: free cmbuf in nvme_exit

This avoid a memory leak in unhotplug nvme device.
Signed-off-by: NLi Qiang <liq3ea@gmail.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

hw/block/nvme.c

@@ -1332,6 +1332,9 @@ static void nvme_exit(PCIDevice *pci_dev)
     g_free(n->cq);
     g_free(n->sq);
 
+    if (n->cmb_size_mb) {
+        g_free(n->cmbuf);
+    }
     msix_uninit_exclusive_bar(pci_dev);
 }