golang/xenlight: Add host-related functionality

golang/xenlight: Add host-related functionality

Add calls for the following host-related functionality:
- libxl_get_max_cpus
- libxl_get_online_cpus
- libxl_get_max_nodes
- libxl_get_free_memory
- libxl_get_physinfo
- libxl_get_version_info

Include Golang versions of the following structs:
- libxl_physinfo as Physinfo
- libxl_version_info as VersionInfo
- libxl_hwcap as Hwcap

Signed-off-by: Ronald Rojas <ronladred@gmail.com>
Reviewed-by:  George Dunlap <george.dunlap@citrix.com>
Acked-by: Ian Jackson <ian.jackson@citrix.com>