qa/suites/rbd: fix sporadic "rx-only direction" test failures
The existing
xmlstarlet sel -t -v '//mirror/peers/peer[1]/uuid')" = ""
test is bogus since a tx-only peer gets added after the remote
rbd-mirror daemon pings the local cluster. It happened to pass most
of the time because xmlstarlet filter just failed on an empty peers
array, producing the wrongly expected empty string by accident.