KVM: selftests: Set per-iteration variables at the start of each iteration

KVM: selftests: Set per-iteration variables at the start of each iteration

Set the per-iteration variables at the start of each iteration instead of
setting them before the loop, and at the end of each iteration.  To ensure
the vCPU doesn't race ahead before the first iteration, simply have the
vCPU worker want for sem_vcpu_cont, which conveniently avoids the need to
special case posting sem_vcpu_cont from the loop.

Link: https://lore.kernel.org/r/20250111003004.1235645-19-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>