kvm-unit-test: x86-64 variant only available on x86-64 hosts

We use the host compiler for x86-64 builds, so ensure it is only available
on x86-64 hosts.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>