nvmeof connector: get ready for nvme-cli 2.0

The nvmeof connector makes calls out to nvme-cli, for example, in create_hostnqn() in os_brick/privileged/nvmeof.py, where our code currently expects that the error_code will map to some errno.

A commit into nvme-cli that changes the exit code to always be either 0 or 1 [0] is in v2.0-rc2, so we can no longer rely on the error_code. Instead, we'll probably have to parse the error message (which, by the way, is sent to stdout; the output sent to stderr is useless).

[0] https://github.com/linux-nvme/nvme-cli/pull/1345/commits/13f74d20e1d9e571368b1d06fe9889a2fdd29849