NetworkManager lost my wired interface

[Paul Smith <paul mad-scientist net>]

Hi all.  Today I upgraded a system from Ubuntu GNOME 16.04 to Ubuntu
GNOME 16.10.  The upgrade went OK, with one exception: my wired
interface has just disappeared from NetworkManager and I can't figure
out how to get it back.  My wireless interface is still available and
works, as do my VPNs.

Also, if I modify the /etc/network/interfaces file and add:

  iface eth0 inet dhcp

then run "sudo ifup eth0" it comes up just fine, so the hardware/kernel
etc. seems to be OK.  I'd like to see if I can get NM to recognize it
again; other programs seem to feel that there's no network on the
system unless NM recognizes it.

I'm not sure where to go to investigate further.  Some info which may
or may not be useful:

$ nmcli device
DEVICE      TYPE      STATE      CONNECTION
wlan0       wifi      connected  MYSSID
lxcbr0      bridge    unmanaged  --
virbr0      bridge    unmanaged  --
eth0        ethernet  unmanaged  --
lo          loopback  unmanaged  --

Trying to change the device to "Managed" doesn't work:

$ sudo nmcli device set eth0 managed yes

$ echo $?
0

$ sudo nmcli device
DEVICE      TYPE      STATE      CONNECTION
wlan0       wifi      connected  MYSSID
lxcbr0      bridge    unmanaged  --
virbr0      bridge    unmanaged  --
eth0        ethernet  unmanaged  --
lo          loopback  unmanaged  --