XenAPI: disable/enable host will be failed when using XenServer

When disable/enable host with XenServer, the api of set_host_enabled()
in XenServer driver needs to get service info for the target host, but
the rpc call of set_host_enabled() did not transfer target host as
parameter, this will cause the api call failed.

This patch was let the API of set_host_enabled() use CONF.host.

Closes-Bug: #1251943

Change-Id: I2b6427df80ee3188d6f7c4eeb4d2e7be300f0126
Merge "XenAPI: disable/enable host will be failed when using XenServer"