initial live migration support

Change-Id: I38c2aee3a7415bb69a099419f6149151c7d6b52c
rewrite gather_host_keys using ssh-keyscan

Change-Id: I4fdbb7436d5e4c4d8864b7427a6b0f15dadb3d4d
handle unknown host errors

Change-Id: Ifb6706939b9cab93b31458cc6d950aadbc331826