VirtualBox network connectivity broken on recent -CURRENT

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.