net/virtio: fix link status always being up

net/virtio: fix link status always being up

The virtio port link status will always be UP, even the port is stopped:

    testpmd> port stop 0
    Stopping ports...
    Checking link statuses...
    Port 0 Link Up - speed 10000 Mbps - full-duplex
    Done

The link status is queried by link_update callback when LSC is disabled.
Which in turn queries the "status" field.  However, the "status" is
read-only. I couldn't think of some proper ways to change the status
without doing device reset.

Instead of doing (the heavy) reset at stop, this patch introduced a flag,
which is set to 1 and 0 on start and stop, respectively. When it's set to
0, the link status is set to DOWN unconditionally.

Fixes: a85786dc816f ("virtio: fix states handling during initialization")
Cc: stable@dpdk.org
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>