shutdown: Add Xen kexec support

In the Xen case, it's the hypervisor which manages kexec. We thus
have to ask it whether a kernel is loaded, instead of relying on
/sys/kernel/kexec_loaded.

src/shared/reboot-util.h

@@ -13,3 +13,5 @@ int read_reboot_parameter(char **parameter);
 int reboot_with_parameter(RebootFlags flags);
 
 int shall_restore_state(void);
+
+bool kexec_loaded(void);