vhost_net: cleanup start/stop condition

Checking vhost device internal state in vhost_net looks like
a layering violation since vhost_net does not
set this flag: it is set and tested by vhost.c.
There seems to be no reason to check this:
caller in virtio net uses its own flag,
vhost_started, to ensure vhost is started/stopped
as appropriate.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
