qemu-iotests: Test the 'base-node' parameter of 'block-stream'

The block-stream command has traditionally used the 'base' parameter
to indicate the image to copy the data from. This test checks that the
'base-node' parameter can also be used for the same purpose.

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