Update vnet hardware flags if link state changes

Update vnet hardware flags if link state changes

Explains a variety of hard-to-diagnose problems with certain Atom and
Denverton NIC types.

I finally tripped over a highly-repeatable failure: home gateway
use-case bitten by refusal to negotiate a DHCP lease for the trunk
port.

The dhcp client won't send pkts unless VNET_HW_INTERFACE_FLAG_LINK_UP
is set on the tx hw interface:

  /* Interface(s) down? */
  if ((hw->flags & VNET_HW_INTERFACE_FLAG_LINK_UP) == 0)
    return;

Change-Id: I17ef2ba7b39078555fa27d2d874a60c67e1530ee
Signed-off-by: Dave Barach <dave@barachs.net>