kvm tools: Use vring_need_event() to determine if interrupt is needed

kvm tools: Use vring_need_event() to determine if interrupt is needed

This patch also fixes fio seq-read hang problem.

   root@guest-kvm:~# cat seq-read.fio
   [seq-read]
   rw=read
   bs=4096
   size=512m
   direct=1
   filename=/dev/vdb

   root@guest-kvm:~# fio seq-read.fio
   random-read: (g=0): rw=read, bs=4K-4K/4K-4K, ioengine=sync, iodepth=1
   Starting 1 process
   Jobs: 1 (f=1): [R] [50.0% done] [0K/0K /s] [0/0 iops] [eta 00m:27s]

Acked-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>