qemu-iotests: Test overlapping stream and commit operations

These test cases check that it's not possible to perform two
block-stream or block-commit operations if there are nodes involved in
both.

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