QNX: Use BarDescriptorDocument API to prepare bar-descriptor for deployment

Patch is refactoring create package step to use new BarDescriptorDocument
class when bar-descriptor.xml is prepared for deployment.

BarDescriptorDocument API is extended to allow this.

Change-Id: If00fba3310c5acf1cc8feefe0cf919aa2a05637e
Reviewed-by: Tobias Nätterlund <tobias.naetterlund@kdab.com>
Reviewed-by: Mehdi Fekari <mfekari@blackberry.com>
Reviewed-by: Nicolas Arnaud-Cormos <nicolas@kdab.com>