virtio-net: properly check the vhost status during status set

Commit 32993698 (vhost: disable on tap link down) tries to disable the vhost
also when the peer's link is down. But the check was not done properly, the
vhost were only started when:

1) peer's link is not down
2) virtio-net has already been started.

Since == have a higher precedence than &&, place a brace to make sure both the
conditions were met then does the check. This fixes the crash when doing a savem
after set the link off which let qemu crash and complains:

virtio_net_save: Assertion `!n->vhost_started' failed.

Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Message-id: 1366972060-21606-1-git-send-email-jasowang@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
