PC platform vm_exit(): fix handling of reboot_on_exit flag

The PC platform code that handles a VM exit should reference the
root_fs global variable (defined in kernel/init.c), which holds a
pointer to the root filesystem, instead of a static variable that
is never set.

platform/pc/service.c

@@ -34,7 +34,7 @@
 #define init_debug(x, ...)
 #endif
 
-static filesystem root_fs;
+extern filesystem root_fs;
 
 #define BOOTSTRAP_REGION_SIZE_KB	2048
 static u8 bootstrap_region[BOOTSTRAP_REGION_SIZE_KB << 10];