win: Enable policy for software SAS

On some systems the policy for enabling the Secure Attention Sequence
might not have been set or has been set to a value which would not allow
the guest agent to perform SSO.
This patch introduces setting the policy before calling 'SendSAS' and
reverting the policy to the previous value (in case it was set before).

Change-Id: Ie2b5f58e461c92e06789d46fa9113441ea48e659
Bug-Url: https://bugzilla.redhat.com/1124263
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>