s390/vfio-ap: Signal eventfd when guest AP configuration is changed

In this patch, an eventfd object is created by the vfio_ap device driver
and used to notify userspace when a guests's AP configuration is
dynamically changed. Such changes may occur whenever:

* An adapter, domain or control domain is assigned to or unassigned from a
  mediated device that is attached to the guest.
* A queue assigned to the mediated device that is attached to a guest is
  bound to or unbound from the vfio_ap device driver. This can occur
  either by manually binding/unbinding the queue via the vfio_ap driver's
  sysfs bind/unbind attribute interfaces, or because an adapter, domain or
  control domain assigned to the mediated device is added to or removed
  from the host's AP configuration via an SE/HMC

The purpose of this patch is to provide immediate notification of changes
made to a guest's AP configuration by the vfio_ap driver. This will enable
the guest to take immediate action rather than relying on polling or some
other inefficient mechanism to detect changes to its AP configuration.

Note that there are corresponding QEMU patches that will be shipped along
with this patch (see vfio-ap: Report vfio-ap configuration changes) that
will pick up the eventfd signal.

Signed-off-by: Rorie Reyes <rreyes@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Tested-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Link: https://lore.kernel.org/r/20250107183645.90082-1-rreyes@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

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