TMF: Add XML state provider schema definition and test file

TMF: Add XML state provider schema definition and test file

This patch adds to the XSD the schema definition for a full XML-defined state
provider. The valid test file is also augmented to use that definition.

Change-Id: Ib75082f04fae69f62f6c7990ee8c92ea4cc042e5
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Signed-off-by: Florian Wininger <florian.wininger@polymtl.ca>
Reviewed-on: https://git.eclipse.org/r/20595
Tested-by: Hudson CI
Reviewed-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
IP-Clean: Alexandre Montplaisir <alexmonthy@voxpopuli.im>