Move boot_set callback backend

Move registration function for the boot_set callback handler and provide
qemu_boot_set so that it can also be used outside the monitor code.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/hw.h b/hw/hw.h
index a424d28..c2c0ac0 100644
--- a/hw/hw.h
+++ b/hw/hw.h
@@ -262,9 +262,10 @@
 
 void qemu_register_reset(QEMUResetHandler *func, void *opaque);
 
-/* handler to set the boot_device for a specific type of QEMUMachine */
+/* handler to set the boot_device order for a specific type of QEMUMachine */
 /* return 0 if success */
-typedef int QEMUBootSetHandler(void *opaque, const char *boot_device);
+typedef int QEMUBootSetHandler(void *opaque, const char *boot_devices);
 void qemu_register_boot_set(QEMUBootSetHandler *func, void *opaque);
+int qemu_boot_set(const char *boot_devices);
 
 #endif