KVM: s390: Fix memory access error detection

Seems that commit 210b1607
(KVM: s390: Removed SIE_INTERCEPT_UCONTROL) lost a hunk when we
reworked our patch queue to rework the async_fp code. We now
ignore faults on the sie instruction (guest accesses non-existing
memory) instead of sending a fault into the guest. This leads to
hang situations with the old virtio transport that checks for
descriptor memory after guest memory. Instead of bailing out this
code now goes wild...
Lets re-add the check.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>