block/amend: Check whether the node exists

block/amend: Check whether the node exists

We should check whether the user-specified node-name actually refers to
a node.  The simplest way to do that is to use bdrv_lookup_bs() instead
of bdrv_find_node() (the former wraps the latter, and produces an error
message if necessary).

Reported-by: Coverity (CID 1430268)
Fixes: ced914d0ab9fb2c900f873f6349a0b8eecd1fdbe
Signed-off-by: Max Reitz <[email protected]>
Message-Id: <20200710095037[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>