[iSCSI] Support Windows Server 2008 direct iSCSI installation

Add yet another ugly hack to iscsiboot.c, this time to allow the user to
inhibit the shutdown/removal of the iSCSI INT13 device (and the network
devices, since they are required for the iSCSI device to function).

On the plus side, the fact that shutdown() now takes flags to
differentiate between shutdown-for-exit and shutdown-for-boot means that
another ugly hack (to allow returning via the PXE stack on BIOSes that
have broken INT 18 calls) will be easier.

I feel dirty.

src/arch/i386/image/elfboot.c

@@ -46,7 +46,7 @@ static int elfboot_exec ( struct image *image ) {
 	/* An ELF image has no callback interface, so we need to shut
 	 * down before invoking it.
 	 */
-	shutdown();
+	shutdown ( SHUTDOWN_BOOT );
 
 	/* Jump to OS with flat physical addressing */
 	__asm__ __volatile__ ( PHYS_CODE ( "call *%%edi\n\t" )