nbd/server: Prepare for per-request filtering of BLOCK_STATUS

nbd/server: Prepare for per-request filtering of BLOCK_STATUS

The next commit will add support for the optional extension
NBD_CMD_FLAG_PAYLOAD during NBD_CMD_BLOCK_STATUS, where the client can
request that the server only return a subset of negotiated contexts,
rather than all contexts.  To make that task easier, this patch
populates the list of contexts to return on a per-command basis (for
now, identical to the full set of negotiated contexts).

Signed-off-by: Eric Blake <eblake@redhat.com>
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v5: fix null dereference on early error [Vladimir], hoist in assertion
from v4 24/24

v4: split out NBDMetaContexts refactoring to its own patch, track
NBDRequests.contexts as a pointer rather than inline