KVM: selftests: Allow specify physical cpu list in demand paging test

Mimic the dirty log test and allow the user to pin demand paging test
tasks to physical CPUs.

Put the help message into a general helper as suggested by Sean.

Signed-off-by: Peter Xu <peterx@redhat.com>
[sean: rebase, tweak arg ordering, add "print" to helper, print program name]
Link: https://lore.kernel.org/r/20230607001226.1398889-1-seanjc@google.com


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