vnc: hextile: do not generate ForegroundSpecified and SubrectsColoured tiles

This violates the RFB specification (section 6.6.4).  It happens to work with
most clients but it's still wrong.

Reported-by: Yaniv Kaul <ykaul@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
