hw/nvme: take a reference on the subsystem on vf realization

Make sure we grab a reference on the subsystem when a VF is realized.
Otherwise, the subsytem will be unrealized automatically when the VFs
are unregistered and unreffed.

This fixes a latent bug but was not exposed until commit 08f6328480
("pcie: Release references of virtual functions"). This was then fixed
(or rather, hidden) by commit c613ad2512 ("pcie_sriov: Do not manually
unrealize"), but that was then reverted (due to other issues) in commit
b0fdaee5d1, exposing the bug yet again.

Cc: qemu-stable@nongnu.org
Fixes: 08f6328480 ("pcie: Release references of virtual functions")
Reviewed-by: Jesper Wendel Devantier <foss@defmacro.it>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>

hw/nvme/ctrl.c

@@ -8841,6 +8841,13 @@ static void nvme_realize(PCIDevice *pci_dev, Error **errp)
          */
         n->params.serial = g_strdup(pn->params.serial);
         n->subsys = pn->subsys;
+
+        /*
+         * Assigning this link (strong link) causes an `object_unref` later in
+         * `object_release_link_property`. Increment the refcount to balance
+         * this out.
+         */
+        object_ref(OBJECT(pn->subsys));
     }
 
     if (!nvme_check_params(n, errp)) {