VirtualBox network connectivity broken on recent -CURRENT

[Don Lewis]

On  3 Jun, Don Lewis wrote:
> It looks like something changed in -CURRENT to break network
> connectivity to VirtualBox guests.  This was last known to work with
> r299139 (May 6th) and is definitely broken with r301229.  The VirtualBox
> port revisions are:
> 	virtualbox-ose-4.3.38_1
> 	virtualbox-ose-kmod-4.3.38
> It looks like there was one change to the VirtualBox on May 9th, but it
> looks unlikely to be the cause of the problem.
> 
> The network settings are:
> 	Attached to: Bridged Adapter
> 	Name: re0
> 	Adapter Type: Paravirtualized Network (virtio-net)
> 	Promiscuous Mode: Deny
> 	MAC Address: [snip]
> Ifconfig says that the interface is up, but I am unable to ping either
> the host or anything else on the LAN from the guest.  It looks like the
> problem is with outbound traffic.  If I attempt to ping the guest, the
> source IP address and MAC address show up in the guest's arp table, but
> ping reports:
> 	ping: sendto: Host is down
> That makes me think that the arp responses from the guest are not
> getting transmitted.  None of the machines involved are running
> firewalls.  If I ping from the guest, I don't see any arp requests on
> the wire and the arp command shows the table entry as incomplete.
> 
> The problem shows up with both FreeBSD -CURRENT and Debian guests.

I see the same behaviour if I set:
	Attached to: NAT
or
	Adapter Type: 82540EM