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