KVM: selftests: Honor "stop" request in dirty ring test

Now that the vCPU doesn't dirty every page on the first iteration for
architectures that support the dirty ring, honor vcpu_stop in the dirty
ring's vCPU worker, i.e. stop when the main thread says "stop".  This will
allow plumbing vcpu_stop into the guest so that the vCPU doesn't need to
periodically exit to userspace just to see if it should stop.

Add a comment explaining that marking all pages as dirty is problematic
for the dirty ring, as it results in the guest getting stuck on "ring
full".  This could be addressed by adding a GUEST_SYNC() in that initial
loop, but it's not clear how that would interact with s390's behavior.

Link: https://lore.kernel.org/r/20250111003004.1235645-10-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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