virtio-net: only delete bh that existed

virtio-net: only delete bh that existed

We delete without check whether it existed during exit. This will lead NULL
pointer deference since it was created conditionally depends on guest driver
status and features. So add a check of existence before trying to delete it.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1383728288-28469-1-git-send-email-jasowang@redhat.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>
(cherry picked from commit fe2dafa02de4f80ab36f6e0f4ddfcd6418c03c49)

Conflicts:

	hw/net/virtio-net.c

*modified to reflect timer function names for 1.6

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>