s390: vfio-ap: base implementation of VFIO AP device driver

Introduces a new AP device driver. This device driver
is built on the VFIO mediated device framework. The framework
provides sysfs interfaces that facilitate passthrough
access by guests to devices installed on the linux host.

The VFIO AP device driver will serve two purposes:

1. Provide the interfaces to reserve AP devices for exclusive
   use by KVM guests. This is accomplished by unbinding the
   devices to be reserved for guest usage from the zcrypt
   device driver and binding them to the VFIO AP device driver.

2. Implements the functions, callbacks and sysfs attribute
   interfaces required to create one or more VFIO mediated
   devices each of which will be used to configure the AP
   matrix for a guest and serve as a file descriptor
   for facilitating communication between QEMU and the
   VFIO AP device driver.

When the VFIO AP device driver is initialized:

* It registers with the AP bus for control of type 10 (CEX4
  and newer) AP queue devices. This limitation was imposed
  due to:

  1. A desire to keep the code as simple as possible;

  2. Some older models are no longer supported by the kernel
     and others are getting close to end of service.

  3. A lack of older systems on which to test older devices.

  The probe and remove callbacks will be provided to support
  the binding/unbinding of AP queue devices to/from the VFIO
  AP device driver.

* Creates a matrix device, /sys/devices/vfio_ap/matrix,
  to serve as the parent of the mediated devices created, one
  for each guest, and to hold the APQNs of the AP devices bound to
  the VFIO AP device driver.

Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Tested-by: Michael Mueller <mimu@linux.ibm.com>
Tested-by: Farhan Ali <alifm@linux.ibm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20180925231641.4954-5-akrowiak@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>