I unchecked the auto-connect option on connection 'Auto SG IP auto' to see if it changes anything.

The behavior is different but satisfying. 
Again, on suspend of the host, NetworkManager disables vboxnet0.
It just does not do DHCP on it.
Again, suspending the resuming the virtual machine repairs the connection.
Summary: still broken and still a regression.
On Ubuntu 14.04 and 14.10, suspending and resuming the host did not interfere with the vboxnet* interfaces.

Thank you for your attention.

Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: unmanaged -> unavailable (reason 'connection-assumed') [10 20 41]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: unavailable -> disconnected (reason 'connection-assumed') [20 30 41]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) starting connection 'vboxnet0'
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 1 of 5 (Device Prepare) scheduled...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 1 of 5 (Device Prepare) started...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: disconnected -> prepare (reason 'none') [30 40 0]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) scheduled...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 1 of 5 (Device Prepare) complete.
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) starting...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: prepare -> config (reason 'none') [40 50 0]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) successful.
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 3 of 5 (IP Configure Start) scheduled.
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) complete.
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 3 of 5 (IP Configure Start) started...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: config -> ip-config (reason 'none') [50 70 0]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 5 of 5 (IPv4 Configure Commit) scheduled...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 3 of 5 (IP Configure Start) complete.
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 5 of 5 (IPv4 Commit) started...
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: ip-config -> ip-check (reason 'none') [70 80 0]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 5 of 5 (IPv4 Commit) complete.
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: ip-check -> secondaries (reason 'none') [80 90 0]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: secondaries -> activated (reason 'none') [90 100 0]
Aug 19 19:04:32 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) successful, device activated.





Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: unmanaged -> unavailable (reason 'connection-assumed') [10 20 41]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: unavailable -> disconnected (reason 'connection-assumed') [20 30 41]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) starting connection 'vboxnet0'
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 1 of 5 (Device Prepare) scheduled...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 1 of 5 (Device Prepare) started...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: disconnected -> prepare (reason 'none') [30 40 0]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) scheduled...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 1 of 5 (Device Prepare) complete.
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) starting...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: prepare -> config (reason 'none') [40 50 0]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) successful.
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 3 of 5 (IP Configure Start) scheduled.
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 2 of 5 (Device Configure) complete.
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 3 of 5 (IP Configure Start) started...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: config -> ip-config (reason 'none') [50 70 0]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 5 of 5 (IPv4 Configure Commit) scheduled...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 3 of 5 (IP Configure Start) complete.
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 5 of 5 (IPv4 Commit) started...
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: ip-config -> ip-check (reason 'none') [70 80 0]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) Stage 5 of 5 (IPv4 Commit) complete.
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: ip-check -> secondaries (reason 'none') [80 90 0]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> (vboxnet0): device state change: secondaries -> activated (reason 'none') [90 100 0]
Aug 19 19:09:42 n55sf-l NetworkManager[9431]: <info> Activation (vboxnet0) successful, device activated.
Aug 19 19:09:42 n55sf-l dbus[767]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service'
Aug 19 19:09:42 n55sf-l systemd[1]: Starting Network Manager Script Dispatcher Service...
Aug 19 19:09:42 n55sf-l dbus[767]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'
Aug 19 19:09:42 n55sf-l systemd[1]: Started Network Manager Script Dispatcher Service.
Aug 19 19:09:42 n55sf-l nm-dispatcher: Dispatching action 'up' for vboxnet0