agent: Perform string sanitization on messages

Messages sent via VirtIO are now checked for being presentable
and additionally filtered for invalid characters due to invalid
encodings.

Change-Id: I499a22e1572b55d1a3a4de7ce6f89ebb130861a8
Bug-Url: https://bugzilla.redhat.com/947014
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>