x86/kvm: hide KVM options from menuconfig when KVM is not compiled

Let KVM_WERROR depend on KVM, so it doesn't show in menuconfig alone.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Message-Id: <20201001112014.9561-1-mcroce@linux.microsoft.com>
Fixes: 4f337faf

 ("KVM: allow disabling -Werror")
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>