vhost-user: control SlaveFsCacheReq with feature
vhost-user: control SlaveFsCacheReq with feature

We import slave_fs_cache mod under vhost-user-slave feature control, but
not the self::slave_fs_cache::SlaveFsCacheReq import.

Signed-off-by: Eryu Guan <269587f4234b28aff6655a07e628f4841352f404@linux.alibaba.com>

rust-vmm/vhost