Added possibility to toggle xml validation

BZ: 184054

It was not possible to enable/disable xml file validation.
The parameter framework only relied on the LIBXML_SCHEMAS flag.

This patchs implements a new constructor for XmlFiles
which allows enabling/disabling xml/xsd check.

Change-Id: I32d220a42bb27f4ce685f07cb194e78e76f44a5d
Signed-off-by: Mattijs Korpershoek <mattijsx.korpershoek@intel.com>