Add nbd tests

[petrutlucian94]

We're adding some tests that exercise the NBD client functionality. The NBD export information can be specified using the following CLI parameters:

• --nbd-export-name
• --nbd-host
• --nbd-port (default: 10809)

If unspecified, the NBD tests will be skipped.

For consistency, we're replacing the WNBD_LOG_LEVEL env variable with a CLI parameter: --log-level.

We'll also address some issues that were brought up by the tests:

• updated srb status in case of nbd errors
• validate IO address on the driver side