nvmf: clear-out qpair during poll group add

There are operations on nvmf which depends on proper values of qpair
attributes which can be intepreted as internal state.
e.g.
nvmf_ctrlr_process_fabrics_cmd execution relies on qpair->ctrlr
spdk_nvmf_qpair_disconnect relies on qpair->disconnect_started

As poll group add is like a registration of qpair into nvmf lets try
to initialize it to a defined and expected state.

Signed-off-by: Jacek Kalwas <jacek.kalwas@intel.com>
Change-Id: I10494e7f70ff58ec5460cab1de8a52fd21cc4a48
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/4479


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>