qapi: copy-on-read filter: add 'bottom' option

qapi: copy-on-read filter: add 'bottom' option

Add an option to limit copy-on-read operations to specified sub-chain
of backing-chain, to make copy-on-read filter useful for block-stream
job.

Suggested-by: Max Reitz <[email protected]>
Suggested-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Signed-off-by: Andrey Shinkevich <[email protected]>
Signed-off-by: Vladimir Sementsov-Ogievskiy <[email protected]>
  [vsementsov: change subject, modified to freeze the chain,
   do some fixes]
Message-Id: <20201216061703[email protected]>
Signed-off-by: Max Reitz <[email protected]>