KVM: selftests: Precisely limit the number of guest loops in mmu_stress_test

Run the exact number of guest loops required in mmu_stress_test instead
of looping indefinitely in anticipation of adding more stages that run
different code (e.g. reads instead of writes).

Reviewed-by: James Houghton <jthoughton@google.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20241128005547.4077116-12-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>