KVM: selftests: Keep dirty_log_test vCPU in guest until it needs to stop

KVM: selftests: Keep dirty_log_test vCPU in guest until it needs to stop

In the dirty_log_test guest code, exit to userspace only when the vCPU is
explicitly told to stop.  Periodically exiting just to check if a flag has
been set is unnecessary, weirdly complex, and wastes time handling exits
that could be used to dirty memory.

Opportunistically convert 'i' to a uint64_t to guard against the unlikely
scenario that guest_num_pages exceeds the storage of an int.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>