Using error_is_set(errp) to check whether a function call failed is
fragile: it breaks when errp is null.  Check perfectly suitable return
values instead when possible.  errp can't be null there now, but this
is more robust and more obviously correct
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>