KVM: selftests: Continuously reap dirty ring while vCPU is running

Continue collecting entries from the dirty ring for the entire time the
vCPU is running.  Collecting exactly once all but guarantees the vCPU will
encounter a "ring full" event and stop.  While testing ring full is
interesting, stopping and doing nothing is not, especially for larger
intervals as the test effectively does nothing for a much longer time.

To balance continuous collection with letting the guest make forward
progress, chunk the interval waiting into 1ms loops (which also makes
the math dead simple).

To maintain coverage for "ring full", collect entries on subsequent
iterations if and only if the ring has been filled at least once.  I.e.
let the ring fill up (if the interval allows), but after that contiuously
empty it so that the vCPU can keep running.

Opportunistically drop unnecessary zero-initialization of "count".

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Link: https://lore.kernel.org/r/20250111003004.1235645-8-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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