test/blockdev: Use regualr RPC socket for mlx5 config

When RPC server is used for configuration, rpc_cmd
function waits 15 seconds to read all replies. If
mlx5 dpdk driver is used on slow machines or in
container, RPC framework_start_init may take more
than 15 seconds to execute. As result, rpc_cmd
exits earlier and output of some comamnds
remains in the pipe. Next call of rpc_cmd may
read wrong data, that leads to malformed json
config. To avoid this problem, redirect RPCs to
a regualr rpc socket.

Signed-off-by: Alexey Marchuk <alexeymar@nvidia.com>
Change-Id: Ibfcf56bb0a7f84f69394846d83746c91a4024b9e
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/16389


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>