vduse: Refactor allocation for vduse virtqueues

Allocate memory for vduse virtqueues one by one instead of
doing one allocation for all of them.

This is a preparation for adding sysfs interface for virtqueues.

Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Message-Id: <20230323053043.35-5-xieyongji@bytedance.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>