s390/vfio-ap: improve reaction to response code 07 from PQAP(AQIC) command

s390/vfio-ap: improve reaction to response code 07 from PQAP(AQIC) command

Let's improve the vfio_ap driver's reaction to reception of response code
07 from the PQAP(AQIC) command when enabling interrupts on behalf of a
guest:

* Unregister the guest's ISC before the pages containing the notification
  indicator bytes are unpinned.

* Capture the return code from the kvm_s390_gisc_unregister function and
  log a DBF warning if it fails.

Suggested-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/20231109164427.460493-4-akrowiak@linux.ibm.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>