kvm tools: disable LTO by default

A bug only seen with LTO enabled was reported by Ron Minnich. Since the issue
appears to be a linker issue, we disable LTO by default until it's more stable.

We can still run LTO builds by setting LTO=1.

Reported-by: Ron Minnich <rminnich@gmail.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>