tests: fix qvirtqueue_kick

vq->avail.idx and vq->avail->ring[] are a 16bit values,
so read and write them with readw()/writew() instead of
readl()/writel().

To read/write a 16bit value with a 32bit accessor works fine
on little-endian CPU but not on big endian CPU.

[An equivalent patch for the writew() calls was also sent by
Zhang Shuai <zhangshuai13@huawei.com>.
--Stefan]

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-id: 1472330054-22607-1-git-send-email-lvivier@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>