libxl: add version_info function

libxl: add version_info function

Xen provides a xen_version hypercall to query the values of several
interesting things (like hypervisor version, commandline used, actual
changeset, etc.). Create a user-friendly and efficient wrapper around
the libxc function to provide values for xl info output.
Since the information is static during the whole runtime, we store
it within the libxl_ctx structure and just deliver the pointer on
subsequent calls.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Acked-by: Vincent Hanquez <vincent.hanquez@eu.citrix.com>