target-i386: Add a marker to end of the region zeroed on reset

Instead of using cpuid_level, use an empty struct as a marker
(like we already did with {start,end}_init_save). This will avoid
accidentaly resetting the wrong fields if we change the field
ordering on CPUX86State.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

target-i386/cpu.c

@@ -2714,7 +2714,7 @@ static void x86_cpu_reset(CPUState *s)
 
     xcc->parent_reset(s);
 
-    memset(env, 0, offsetof(CPUX86State, cpuid_level));
+    memset(env, 0, offsetof(CPUX86State, end_reset_fields));
 
     tlb_flush(s, 1);
 

target-i386/cpu.h

@@ -1108,6 +1108,7 @@ typedef struct CPUX86State {
     CPU_COMMON
 
     /* Fields from here on are preserved across CPU reset. */
+    struct {} end_reset_fields;
 
     /* processor features (e.g. for CPUID insn) */
     uint32_t cpuid_level;