s390/vfio-ap: add status attribute to AP queue device's sysfs dir

This patch adds a sysfs 'status' attribute to a queue device when it is
bound to the vfio_ap device driver. The field displays a string indicating
the status of the queue device:

Status String:  Indicates:
-------------   ---------
"assigned"      the queue is assigned to an mdev, but is not in use by a
                KVM guest.
"in use"        the queue is assigned to an mdev and is in use by a KVM
                guest.
"unassigned"    the queue is not assigned to an mdev.

The status string will be displayed by the 'lszcrypt' command if the queue
device is bound to the vfio_ap device driver.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
[akrowiak@linux.ibm.com: added check for queue in use by guest]
Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>