xenapi: remove unnecessary json decoding of injected_files
xenapi: remove unnecessary json decoding of injected_files

The API is responsible for decoding JSON an XML content. Also, the
code decoded results weren't used since the code below it would use
the original data anyway.

Also, add a unit test for injected_files in the xenapi driver.

Change-Id: Ie96b301b0948f92dd90060eacba0db9ae940b952

n0ano/ganttclient