hmp: Make block_stream set an explicit job ID

A job ID is always required in order to create a block job on a
non-root node. The default ID (obtained with bdrv_get_device_name())
is otherwise empty in this scenario and the job cannot be created.

The HMP block_stream command doesn't set a job ID and therefore it
doesn't allow streaming to intermediate nodes. One solution is to add
an extra parameter to set a job ID. The other solution is to simply
use the node name passed to block_stream as job ID. This won't work
if it's automatically generated (because it contains a '#') but is
otherwise simple enough for all other cases.

This way 'block_stream node3' will create a job with the ID 'node3'
and the good old 'block_stream virtio0' will keep the previous
behaviour and use 'virtio0' for the job ID.

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