vfio-pci: Use mutex around open, release, and remove

Serializing open/release allows us to fix a refcnt error if we fail
to enable the device and lets us prevent devices from being unbound
or opened, giving us an opportunity to do bus resets on release.  No
restriction added to serialize binding devices to vfio-pci while the
mutex is held though.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci_private.h

@@ -55,7 +55,7 @@ struct vfio_pci_device {
 	bool			bardirty;
 	bool			has_vga;
 	struct pci_saved_state	*pci_saved_state;
-	atomic_t		refcnt;
+	int			refcnt;
 	struct eventfd_ctx	*err_trigger;
 };