KVM: selftests: Move dirty logging functions to memstress.(c|h)

Move some helper functions from dirty_log_perf_test.c to the memstress
library so that they can be used in a future commit which tests page
splitting during dirty logging.

Reviewed-by: Vipin Sharma <vipinsh@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Link: https://lore.kernel.org/r/20230131181820.179033-2-bgardon@google.com


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