migration: pre_save return int

Modify the pre_save method on VMStateDescription to return an int
rather than void so that it potentially can fail.

Changed zillions of devices to make them return 0; the only
case I've made it return non-0 is hw/intc/s390_flic_kvm.c that already
had an error_report/return case.

Note: If you add an error exit in your pre_save you must emit
an error_report to say why.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20170925112917.21340-2-dgilbert@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

slirp.c

@@ -1195,11 +1195,13 @@ struct sbuf_tmp {
     uint32_t roff, woff;
 };
 
-static void sbuf_tmp_pre_save(void *opaque)
+static int sbuf_tmp_pre_save(void *opaque)
 {
     struct sbuf_tmp *tmp = opaque;
     tmp->woff = tmp->parent->sb_wptr - tmp->parent->sb_data;
     tmp->roff = tmp->parent->sb_rptr - tmp->parent->sb_data;
+
+    return 0;
 }
 
 static int sbuf_tmp_post_load(void *opaque, int version)
@@ -1292,7 +1294,7 @@ typedef struct SS_FamilyTmpStruct {
 #define SS_FAMILY_MIG_IPV6 10 /* Linux */
 #define SS_FAMILY_MIG_OTHER 0xffff
 
-static void ss_family_pre_save(void *opaque)
+static int ss_family_pre_save(void *opaque)
 {
     SS_FamilyTmpStruct *tss = opaque;
 
@@ -1303,6 +1305,8 @@ static void ss_family_pre_save(void *opaque)
     } else if (tss->parent->ss.ss_family == AF_INET6) {
         tss->portable_family = SS_FAMILY_MIG_IPV6;
     }
+
+    return 0;
 }
 
 static int ss_family_post_load(void *opaque, int version_id)