net/virtio-user: move eventfd open/close into init/uninit

Originally, eventfd is opened when initializing each vq; and gets closded
in virtio_user_stop_device().

To make it possible to initialize intr_handle struct in init() in following
patch, we put the open() of all eventfds into init(); and put the close()
into uninit().

Suggested-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>