vga: fix region calculation
    
    Typically the scanline length and the line offset are identical.  But
    in case they are not our calculation for region_end is incorrect.  Using
    line_offset is fine for all scanlines, except the last one where we have
    to use the actual scanline length.
    
    Fixes: CVE-2018-7550
    Reported-by: Ross Lagerwall <ross.lagerwall@citrix.com>
    Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
    Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
    Tested-by: Ross Lagerwall <ross.lagerwall@citrix.com>
    Message-id: 20180309143704.13420-1-kraxel@redhat.com
    (cherry picked from commit 7cdc61becd095b64a786b2625f321624e7111f3d)
    Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>