do not call vhost_net_cleanup() on running net from char user event

do not call vhost_net_cleanup() on running net from char user event

Buglink: https://launchpad.net/bugs/1823458
Currently, a user CHR_EVENT_CLOSED event will cause net_vhost_user_event()
to call vhost_user_cleanup(), which calls vhost_net_cleanup() for all
its queues.  However, vhost_net_cleanup() must never be called like
this for fully-initialized nets; when other code later calls
vhost_net_stop() - such as from virtio_net_vhost_status() - it will try
to access the already-cleaned-up fields and fail with assertion errors
or segfaults.

The vhost_net_cleanup() will eventually be called from
qemu_cleanup_net_client().

Signed-off-by: Dan Streetman <ddstreet@canonical.com>
Message-Id: <20190416184624.15397-3-dan.streetman@canonical.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>