Support communicating with Xen API over https

[chqr]

For security reasons, we'd like to avoid transmitting XenServer credentials in the clear. It looks like under the hood this project is using the Go standard library's http implementation, which supports https.

Are there any objections to adding a remote_url config flag, so that users can specify http/https according to their needs? It looks like the value of remote_url could simply be passed in to XenAPIClient, and could default to "http://${remote_host}" (the current behavior).