vfio: Split virqfd into a separate module for vfio bus drivers

An unintended consequence of commit 42ac9bd18d ("vfio: initialize
the virqfd workqueue in VFIO generic code") is that the vfio module
is renamed to vfio_core so that it can include both vfio and virqfd.
That's a user visible change that may break module loading scritps
and it imposes eventfd support as a dependency on the core vfio code,
which it's really not.  virqfd is intended to be provided as a service
to vfio bus drivers, so instead of wrapping it into vfio.ko, we can
make it a stand-alone module toggled by vfio bus drivers.  This has
the additional benefit of removing initialization and exit from the
core vfio code.

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

drivers/vfio/vfio.c

@@ -1552,11 +1552,6 @@ static int __init vfio_init(void)
 	if (ret)
 		goto err_cdev_add;
 
-	/* Start the virqfd cleanup handler used by some VFIO bus drivers */
-	ret = vfio_virqfd_init();
-	if (ret)
-		goto err_virqfd;
-
 	pr_info(DRIVER_DESC " version: " DRIVER_VERSION "\n");
 
 	/*
@@ -1569,8 +1564,6 @@ static int __init vfio_init(void)
 
 	return 0;
 
-err_virqfd:
-	cdev_del(&vfio.group_cdev);
 err_cdev_add:
 	unregister_chrdev_region(vfio.group_devt, MINORMASK);
 err_alloc_chrdev:
@@ -1585,7 +1578,6 @@ static void __exit vfio_cleanup(void)
 {
 	WARN_ON(!list_empty(&vfio.group_list));
 
-	vfio_virqfd_exit();
 	idr_destroy(&vfio.group_idr);
 	cdev_del(&vfio.group_cdev);
 	unregister_chrdev_region(vfio.group_devt, MINORMASK);