kvm: Remove unreachable code in kvm_dirty_ring_reaper_thread()

kvm: Remove unreachable code in kvm_dirty_ring_reaper_thread()

The code at the tail end of the loop in kvm_dirty_ring_reaper_thread()
is unreachable, because there is no way for execution to leave the
loop. Replace it with a g_assert_not_reached().

(The code has always been unreachable, right from the start
when the function was added in commit b4420f198dd8.)

Resolves: Coverity CID 1547687
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240815131206.3231819-3-peter.maydell@linaro.org