tools: hv: set allow-hotplug for VF on Ubuntu

On HyperV, the VF interface can be offered by a host at any time.
Mark the VF interface as hotplug, to make sure it will be brought up
automatically when it is registered.

Signed-off-by: Simon Xiao <sixiao@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>