Revert "Remove contra productive _string_check"

This reverts commit b06a6c4c8e61013b7461c6fc946c1530ae47584e.
The previous commit breaks the guest agent badly because the
string type is required to be unicode, this code previously was
converting it most of the times to unicode, however in some cases
it won't do so and causes the data to be an ascii string or even
replaces valid characters even though it should not.

The attempt for fixing this issue will be addressed by a new patch.

Change-Id: I47176ac60e069e7d557312d7047d9bc4b15ac768
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Adding unit tests

Change-Id: Ibf5c64da1904510ca2f8fb5c91756a3133b79fb3
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Fixed Makefile tabs

Change-Id: I9c7eae9b25a9ea088dd08722090744bca126f320
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Merging Win32 and Linux unittests and add Message validation

Change-Id: I66df122741dbdfb14d7ea080672dfbaf10f87cd6
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>