net/virtio-user: fix wrongly get/set features

net/virtio-user: fix wrongly get/set features

Before the commit 86d59b21468a ("net/virtio: support LRO"), features
in virtio PMD, is decided and properly set at device initialization
and will not be changed. But afterward, features could be changed in
virtio_dev_configure(), and will be re-negotiated if it's changed.

In virtio-user, device features is obtained at driver probe phase
only once, but we did not store it. So the added feature bits in
re-negotiation will fail.

To fix it, we store it down, and will be used to feature negotiation
either at device initialization phase or device configure phase.

Fixes: e9efa4d93821 ("net/virtio-user: add new virtual PCI driver")
Cc: stable@dpdk.org
Signed-off-by: Jianfeng Tan <jianfeng.tan@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>