kvm tools: Default guest cpu count to host cpu count

If user haven't specified cpu count for the guest, use
the amount of online cpus on the host.

Tested-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>