agent: Reworking the data filtering for the guest agent

This patchset is oriented on http://gerrit.ovirt.org/#/c/16652

We're now filtering properly invalid unicode characters and invalid
characters. Which would cause VDSM to build invalid XML data.

Change-Id: I3a4347350d791f4df0b26b1cf81d0ca4f9656981
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>