Build time check of OFD lock is not sufficient and can cause image open
errors when the runtime environment doesn't support it.

Add a helper function to probe it at runtime, additionally. Also provide
a qemu_has_ofd_lock() for callers to check the status.
Signed-off-by: NFam Zheng <famz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>