Add error message for loading snapshot without VM state

It already fails, but it didn't tell the user why.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

savevm.c

@@ -2021,6 +2021,8 @@ int load_vmstate(const char *name)
     if (ret < 0) {
         return ret;
     } else if (sn.vm_state_size == 0) {
+        error_report("This is a disk-only snapshot. Revert to it offline "
+            "using qemu-img.");
         return -EINVAL;
     }