[Enhancement] ensure_sshd method on Guest instances

[lingxiaoyang]

As discussed a little bit here, some setups such as ensuring sshd started could be used elsewhere and depend on Linux distributions. It may be better to move these common "ensures" to Guest instances. Then, for example, the provisioner can simply call self.guest.ensure_sshd() before provisioning.

Implementation details need to be discussed.

[ghost]

This issue will become irrelevant with #94