qxl: Fix QXLRam initialisation.

The qxl driver expect NULL for QXLRam.memory_configs, but this is never
initialized.

If memory is set to 0xc2c2.., it leads to a spice-critical error when
trying to start qxl.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
