winrtrunner: Accept backslashes as well when looking for manifest file

The mapping file might contain paths having slashes as well as
backslashes as separators. When looking for the manifest file in this
list the kind of separators used should not matter.

Change-Id: I80e36f3bc4d3f80c34291bfa845b345f5638f88d
Reviewed-by: Andrew Knight <andrew.knight@digia.com>