[libvirt] crash in libxlDoMigrateDstReceive in error case

Jim, while fixing and debugging virNetSocketNewListenTCP in master, I had breakpoints in virNetSocketNewListenTCP and virNetSocketNew. After starting a migration the receiving libvirtd went into both functions. I hit continue a 4 times, but forgot to do it for the fifth call to let it proceed. libvirtd was stuck there for a long time. As a result, when leaving virNetSocketNew, I got a crash in libxlDoMigrateDstReceive -> libxlDomainStartRestore -> libxlDomainStart because args->conn->privateData is NULL. This means 'driver' becomes NULL and things fall apart. I have no checked where privateData is set. I think some place should catch the broken connection and stop the incoming migration. Olaf