block: Add QMP support for streaming to an intermediate layer

This patch makes the 'device' parameter of the 'block-stream' command
accept a node name that is not a root node. The presence of this
feature can't be directly tested with introspection; soon we'll
introduce a 'base-node' parameter whose presence can be checked for
this purpose.

In addition to that, operation blockers will be checked in all
intermediate nodes between the top and the base node.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
