migration wrong handling of KVM_GET_DIRTY_LOG ioctl

In the code below kvm_vm_ioctl(...) can return --errno != -1 from ioctl call,  but return only checks for -1. 
Found during KVM-ARM migration which apperead to go through but was actually failing getting 
memslot dirty bitmap.

static int kvm_physical_sync_dirty_bitmap(....)
{
 ....
 if(kvm_vm_ioctl(s, KVM_GET_DIRTY_LOG, &d) == -1) {
   - err out
 }
 ... continue
}