The variable base_gfn is initialized but never used
otherwise, so remove the unused variable.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch)
Acked-by: Xiantao Zhang <xiantao.zhang@intel.com>
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>