Prefer to use the tcg accelarator if it is available: This is our only
real smoke test for tcg, and fast enough to use it for that.

Fixes: 480bc11e ("boot-serial-test: fallback to kvm accelerator")
Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>