testutil: Fix data race between instance addition & stats events
testutil: Fix data race between instance addition & stats events

When running the controller unit test testutil.SendStats() is trying to
read from a structure that SsntpTestClient.handleAttachVolume() is
actively updating: the list of volumes in the instance.

The solution is to use the existing lock for reading the instance
structure as well as when writing.

Fixes: #868

Signed-off-by: Rob Bradford <c49e1dc14cd52e2644816db47123896e0ac79025@intel.com>

rbradford/ciao,markdryan/ciao,matthewrsj/ciao,mcastelino/ciao,tpepper/ciao,markdryan/ciao,mcastelino/ciao,ciao-project/ciao,wcwxyz/ciao,mcastelino/ciao,erick0z/ciao,matthewrsj/ciao,rbradford/ciao,kaccardi/ciao,obedmr/ciao,ciao-project/ciao,wcwxyz/ciao,wcwxyz/ciao,mrkz/ciao,tpepper/ciao,01org/ciao,obedmr/ciao,tpepper/ciao,mrkz/ciao,erick0z/ciao,01org/ciao,kaccardi/ciao,obedmr/ciao,mrkz/ciao,matthewrsj/ciao,erick0z/ciao