s390x/event-facility: fix error propagation

s390x/event-facility: fix error propagation

We currently check (by error) if the passed-in Error pointer errp
is non-null and return after realizing the first child of the
event facility in that case. Symptom is that 'virsh shutdown'
does not work, as the sclpquiesce device is not realized.

Fix this by (correctly) checking the local Error err.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Fixes: 3d508334dd2c ("s390x/event-facility: Fix realize() error API violations")
Message-Id: <20200121095506.8537-1-cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>