nbd/client: Split out nbd_send_meta_query()

nbd/client: Split out nbd_send_meta_query()

Refactor nbd_negotiate_simple_meta_context() to pull out the
code that can be reused to send a LIST request for 0 or 1 query.
No semantic change.  The old comment about 'sizeof(uint32_t)'
being equivalent to '/* number of queries */' is no longer
needed, now that we are computing 'sizeof(queries)' instead.

Signed-off-by: Eric Blake <[email protected]>
Reviewed-by: Richard W.M. Jones <[email protected]>
Message-Id: <20190117193658[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>