CONFIG_XILINX_VIRTEX is not available for thedefaultarchitecture x86.
Result is shown for architecture ppc

CONFIG_XILINX_VIRTEX (not configurable)

modulename: virtex.ko

configname: CONFIG_XILINX_VIRTEX

Linux Kernel Configuration
└─>IBM 4xx options
└─>XILINX_VIRTEX
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
source code:
arch/ppc/platforms/4xx/virtex.c

is selected by
CONFIG_FB_XILINX