KVM: selftests: Set threads CPU affinity before doing work in hardware disable test

Affine worker threads to their target pCPU set *before* doing any work in
the hardware disable test.  When running a preemptible kernel on a NUMA
system with a large number of CPUs (100+ of CPUs), waiting to set the
affinity of the grandchildren until after the grandchild is fully spun up
results in total test runtimes of over a minute, presumably due to
cross-node NUMA penalties and scheduler lock contention.

E.g. from the perspective of the caller, when waiting to affine the tasks,
pthread_create() takes ~100x longer.

Link: https://patch.msgid.link/20260731195612.2697986-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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