Fix migration on new KVMs

New KVMs (0.12.1.2-el6 and 0.13.5 tested) exit immediately after
unsuccessful network connection when they are in "-incoming" mode. The
simple check netutils.TcpPing causes remote kvm to exit so the migration
will always fail. This check is also redundant by the way as if the
network port is closed migration will fail with guarantee. This commit
removes this check.

Signed-off-by: Vitaly Kuznetsov <vitty@altlinux.ru>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>