acpi: fix buffer overrun on migration

ich calls acpi_gpe_init with length ICH9_PMIO_GPE0_LEN so
ICH9_PMIO_GPE0_LEN/2 bytes are allocated, but then the full
ICH9_PMIO_GPE0_LEN bytes are migrated.

As a quick work-around, allocate twice the memory.
We'll probably want to tweak code to avoid
migrating the extra ICH9_PMIO_GPE0_LEN/2 bytes,
but that is a bit trickier to do without breaking
migration compatibility.
Tested-by: N"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Reported-by: N"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/acpi/core.c

@@ -625,8 +625,12 @@ void acpi_pm1_cnt_reset(ACPIREGS *ar)
 void acpi_gpe_init(ACPIREGS *ar, uint8_t len)
 {
     ar->gpe.len = len;
-    ar->gpe.sts = g_malloc0(len / 2);
-    ar->gpe.en = g_malloc0(len / 2);
+    /* Only first len / 2 bytes are ever used,
+     * but the caller in ich9.c migrates full len bytes.
+     * TODO: fix ich9.c and drop the extra allocation.
+     */
+    ar->gpe.sts = g_malloc0(len);
+    ar->gpe.en = g_malloc0(len);
 }
 
 void acpi_gpe_reset(ACPIREGS *ar)