remove use of get_gendisk in blkfront
gen_gendisk is no long exported by 2.6 kernel. this patch remove the 
call of get_gendisk by saving the gd point in blkfront_info in vbd.c so 
that blkfront can be built as kerenl module.

Signed-off-by: Xiaofeng Ling <xiaofeng.ling@intel.com>