efi_loader: LocateHandle should return EFI_NOT_FOUND if none found

Spotted this debugging OpenBSD's bootloader in qemu.  (Wouldn't really
fix anything, the problem was not having any disks, but we should
probably return the correct error code.)

Signed-off-by: Rob Clark <robdclark@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>