nbd: BLOCK_STATUS for standard get_block_status function: server part

nbd: BLOCK_STATUS for standard get_block_status function: server part

Minimal realization: only one extent in server answer is supported.

Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Message-Id: <20180312152126[email protected]>
Reviewed-by: Eric Blake <[email protected]>
[eblake: tweak whitespace, move constant from .h to .c, improve
logic of check_meta_export_name, simplify nbd_negotiate_options
by doing more in nbd_negotiate_meta_queries]
Signed-off-by: Eric Blake <[email protected]>