core:OsInfo-related changes

This patch addresses OsInfo parts of the code.

With the split of DisplayType enum to DisplayType and GraphicsType,
OsInfo code had to be changed to reflect this.

OsInfo queries related to DisplayType (getDisplayTypes,..) were changed
so that they now contain information about GraphicsTypes as well.

small todo:
 - consider renaming OsInfo key display.protocol to something more sane.

Change-Id: If0c7126308ab4e5e5418d804a8550a50e9994894
Signed-off-by: Frantisek Kobzik <fkobzik@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1033547