[sled-agent] Fail with helpful error message if kernel does not support xde driver

[bnaecker]

@jmpesp Has hit an issue with version skew between the stock helios-dev publisher and the publisher that provides the modified kernel that the xde driver expects, to be able to register itself with DLD. This results in errors like:

{
  "msg": "request completed",
  "v": 0,
  "name": "bootstrap-agent",
  "level": 30,
  "time": "2022-05-12T20:22:59.083099687Z",
  "hostname": "dinnerbone",
  "pid": 623,
  "uri": "/start_sled",
  "method": "PUT",
  "req_id": "7895d08c-152e-4417-8551-78f812f5a640",
  "remote_addr": "[fdb0:1b:21c1:ffe0::1]:65275",
  "local_addr": "[fdb0:1b:21c1:ffe0::1]:12346",
  "component": "dropshot",
  "error_message_external": "Internal Server Error",
  "error_message_internal": "Error starting sled agent: Could not start sled agent server: Error managing guest networking: Failure interacting with the OPTE ioctl(2) interface: ioctl SetXdeUnderlay failed: unexpected errno: 48",
  "response_code": "500"
}

That "unexpected errno" is ENOTSUP, which means that the registration with DLD failed, because DLD doesn't understand who xde is. This should be detected, and a more useful error message printed out, describing that the kernel does not support the xde driver and how it can be updated so that's the case.

[bnaecker]

Closed by #1070