Storage: add API for performing the NVMe discovery

[harshitap26]

No API is available under the Host facing API for performing the NVMe discover command. We need the NVMe discover command to find the target NQN, which is required in the NVMe connect command.

[benlwalker]

I believe this is not the correct approach. Instead, when provisioning a back-end volume, specifying a discovery service instead of the real subnqn/ip/port is the right way to handle discovery. In other words, you say "I want to connect to the volume that this discovery service is telling me about" when you create the back end volume.

The reason to do it the way I suggest is because the connection parameters can actually migrate over time, so the xPU needs to remain subscribed to the discovery service indefinitely to track the path changes. You can't just ask the discovery service one time and then use those same parameters forever, in the general case.

Recommend closing this issue and opening one targeted at the nvmf backend to support connect via discovery.

[glimchb]

@benlwalker by doing so you assuming xPU vendors implement discovery service that runs all the time and monitors the network... one can use CDC instead of DDC or SPDK start discovery, but this is not all the use cases...
Maybe API for discovery is not instead of discovery but in addition...

[benlwalker]

I think even in the case that the discovery service isn't continually running and paths never move around, you'd still use the discovery service during the provisioning of the back-end volume rather than having dedicated calls to query it.