virtio_fs: fix the hash table using in virtio_fs_enqueue_req()

The original commit be2ff42c ("fuse: Use hash table to link
processing request") converted fuse_pqueue->processing to a hash table,
but virtio_fs_enqueue_req() was not updated to use it correctly.
So use fuse_pqueue->processing as a hash table, this make the code
more coherent

Co-developed-by: Fushuai Wang <wangfushuai@baidu.com>
Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>