kvm tools: Update max VCPU limit

Use the newly introduced KVM_CAP_MAX_VCPUS to determine max VCPU count
the kernel side supports.

Using more VCPUs than the recommended limit will issue a warning, using more
than the absolute maximum will limit it down to the absolute maximum.

Note this patch contains a hack to define KVM_CAP_MAX_VCPUS and should be
removed when the kernel patch is merged with mainline.

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>