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: ced914d0
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-Id: <20200710095037.10885-1-mreitz@redhat.com>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>