KVM: selftests: Randomize dirty_log_test's delay before reaping the bitmap

KVM: selftests: Randomize dirty_log_test's delay before reaping the bitmap

In the dirty log test, randomize the delay before the initial call to get
the dirty log bitmap for a given iteration, so that the amount of memory
dirtied by the guest varies from iteration to iteration, and so that the
user can effectively control the duration (by increasing the interval).

Always waiting 1ms effectively hides a KVM RISC-V bug as the test reaps the
dirty bitmap before the guest has a chance to trigger the problematic flow
in KVM.

Reported-by: Wu Fei <wu.fei9@sanechips.com.cn>
Closes: https://lore.kernel.org/all/202605111130.64BBUXDN013040@mse-fl2.zte.com.cn
Cc: Wu Fei <atwufei@163.com>
Link: https://patch.msgid.link/20260522170230.3518669-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>