s390x/kvm: Log unmanageable external interruptions

Interception code 0x14 only drops to userspace when an unmanageable
external interruption interception occured (e.g. if the External New
PSW does not disable external interruptions). Instead of bailing out
via the default handler, it is better to inform the user with a
proper error message that also includes the bad PSW, and to stop
the affected CPU with a panic event instead.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
