loss of connectivity on upstream VM restart

[xaki23]

when the upstream VM of a mirage firewall gets restarted, the mfw doesnt recover from that and needs to be restarted as well.

repro steps:

• set up a network chain like internet <...> linuxA <-> miragefw <-> linuxB
• ping something external from linuxB
• qvm-kill linuxA (or run poweroff inside it)
• qvm-start linuxA
• note even after linuxA is up again, linuxB will not be able to reach across the chain
• restart miragefw
• connections work again

a linux FW in the middle position will recover from temporary loss of its upstream interface. there doesnt seem to be anything relevant being logged on the mfw console.