lguest: suppress interrupts for single insn, not range.

The last patch reduced our interrupt-suppression region to one address,
so simplify the code somewhat.

Also, remove the obsolete undefined instruction ranges and the comment
which refers to lguest_guest.S instead of head_32.S.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/include/linux/lguest.h b/include/linux/lguest.h
index 9962c6b..6db19f3 100644
--- a/include/linux/lguest.h
+++ b/include/linux/lguest.h
@@ -61,8 +61,8 @@
 	u32 tsc_khz;
 
 /* Fields initialized by the Guest at boot: */
-	/* Instruction range to suppress interrupts even if enabled */
-	unsigned long noirq_start, noirq_end;
+	/* Instruction to suppress interrupts even if enabled */
+	unsigned long noirq_iret;
 	/* Address above which page tables are all identical. */
 	unsigned long kernel_address;
 	/* The vector to try to use for system calls (0x40 or 0x80). */