qxl: fix local renderer

The local spice renderer assumes the primary surface is located at the
start of the "ram" bar.  This used to be a requirement in qxl hardware
revision 1.  In revision 2+ this is relaxed.  Nevertheless guest drivers
continued to use the traditional location, for historical and backward
compatibility reasons.  The qxl kms driver doesn't though as it depends
on qxl revision 4+ anyway.

Result is that local rendering is hosed for recent linux guests, you'll
get pixel garbage with non-spice ui (gtk, sdl, vnc) and when doing
screendumps.  Fix that by doing a proper mapping of the guest-specified
memory location.

https://bugzilla.redhat.com/show_bug.cgi?id=948717

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
