vhost: remove useless volatile
vhost: remove useless volatile

last_used_idx is a local var, there is no need to decorate it
by "volatile".

Signed-off-by: Zhihong Wang <efad9c89b3a8db7900a2572fdee08ebbff3cfc6b@intel.com>
Reviewed-by: Jianbo Liu <ad48e27d4475ec2c10ba93ba465af79f05639344@linaro.org>
Tested-by: Maxime Coquelin <9a2667dee3b90866bbc9c2696cff2b7d5616c95a@redhat.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk