migration: Don't allow migration if vm is in POSTMIGRATE

The following steps will cause qemu assertion failure:
- pause vm by executing 'virsh suspend'
- create external snapshot of memory and disk using 'virsh snapshot-create-as'
- doing the above operation again will cause qemu crash

The backtrace looks like:
    at /build/qemu-5.0/migration/savevm.c:1401
    at /build/qemu-5.0/migration/savevm.c:1453

When the first migration completes, bs->open_flags will set BDRV_O_INACTIVE
flag by bdrv_inactivate_all(), and during the second migration the
bdrv_inactivate_recurse assert that the bs->open_flags is already
BDRV_O_INACTIVE enabled which cause crash.

As Vladimir suggested, this patch makes migrate_prepare check the state of vm and
return error if it is in RUN_STATE_POSTMIGRATE state.

Signed-off-by: Tuguoyi <tu.guoyi@h3c.com>
Message-Id: <6b704294ad2e405781c38fb38d68c744@h3c.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reported-by: Li Zhang <li.zhang@cloud.ionos.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@cloud.ionos.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

migration/migration.c

@@ -2102,6 +2102,12 @@ static bool migrate_prepare(MigrationState *s, bool blk, bool blk_inc,
         return false;
     }
 
+    if (runstate_check(RUN_STATE_POSTMIGRATE)) {
+        error_setg(errp, "Can't migrate the vm that was paused due to "
+                   "previous migration");
+        return false;
+    }
+
     if (migration_is_blocked(errp)) {
         return false;
     }