virtio-net: avoid call tap_enable when there's only one queue

virtio-net: avoid call tap_enable when there's only one queue

We call tap_enable() even if for multiqueue is not enabled. This is
wrong since it should be used for multiqueue codes to enable a
disabled queue. Fixing this by only calling this when multiqueue is
used.

Fixes: 16dbaf905b72 ("tap: support enabling or disabling a queue")
Reported-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Tested-by: Andrew Baumann <Andrew.Baumann@microsoft.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>