libxc: add API for kexec hypercall

libxc: add API for kexec hypercall

Add xc_kexec_exec(), xc_kexec_get_ranges(), xc_kexec_load(), and
xc_kexec_unload().  The load and unload calls require the v2 load and
unload ops.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Tested-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Don Slutz <dslutz@verizon.com>
Tested-by: Don Slutz <dslutz@verizon.com>
Acked-by: Keir Fraser <keir@xen.org>