Allow mdev drivers to directly create the vfio_device (v3)

[Christoph Hellwig <hch@lst.de>]

This is my alternative take on this series from Jason:

https://lore.kernel.org/dri-devel/87czsszi9i.fsf@redhat.com/T/

The mdev/vfio parts are exactly the same, but this solves the driver core
changes for the direct probing without the in/out flag that Greg hated,
which cause a little more work, but probably make the result better.

Original decription from Jason below:

The mdev bus's core part for managing the lifecycle of devices is mostly
as one would expect for a driver core bus subsystem.

However instead of having a normal 'struct device_driver' and binding the
actual mdev drivers through the standard driver core mechanisms it open
codes this with the struct mdev_parent_ops and provides a single driver
that shims between the VFIO core's struct vfio_device and the actual
device driver.

Instead, allow mdev drivers implement an actual struct mdev_driver and
directly call vfio_register_group_dev() in the probe() function for the
mdev. Arrange to bind the created mdev_device to the mdev_driver that is
provided by the end driver.

The actual execution flow doesn't change much, eg what was
parent_ops->create is now device_driver->probe and it is called at almost
the exact same time - except under the normal control of the driver core.

Ultimately converting all the drivers unlocks a fair number of additional
VFIO simplifications and cleanups.

Changes since v1:
 - avoid warning spam for successful probes
 - improve the probe_count protection