selftests: breakpoints: fix computation of test plan

The computation of the test plan uses the available_cpus bitset
before calling sched_getaffinity to fill it in.  The resulting
plan is bogus, fix it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>