Coverity doesn't like that most callers of vfio_set_irq_signaling() check
the return value and doesn't understand the equivalence of testing the
error pointer instead.  Test the return value consistently.

Reported-by: Coverity (CID 1402783)
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Message-Id: <156209642116.14915.9598593247782519613.stgit@gimli.home>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>