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

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 08f632848008
("pcie: Release references of virtual functions"). This was then fixed
(or rather, hidden) by commit c613ad25125b ("pcie_sriov: Do not manually
unrealize"), but that was then reverted (due to other issues) in commit
b0fdaee5d1ed, exposing the bug yet again.

Cc: qemu-stable@nongnu.org
Fixes: 08f632848008 ("pcie: Release references of virtual functions")
Reviewed-by: Jesper Wendel Devantier <foss@defmacro.it>
Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
(cherry picked from commit 6651f8f2e5051f6750c2534ab3151339b3c476a2)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>