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/systemctl/systemctl-compat-shutdown.c

@@ -4,6 +4,7 @@
 
 #include "alloc-util.h"
 #include "pretty-print.h"
+#include "reboot-util.h"
 #include "systemctl-compat-shutdown.h"
 #include "systemctl-sysv-compat.h"
 #include "systemctl.h"

src/systemctl/systemctl.c

@@ -17,6 +17,7 @@
 #include "path-util.h"
 #include "pretty-print.h"
 #include "process-util.h"
+#include "reboot-util.h"
 #include "rlimit-util.h"
 #include "sigbus.h"
 #include "signal-util.h"