KVM: selftests: Create a separate dirty bitmap per slot

The calculation to get the per-slot dirty bitmap was incorrect leading
to a buffer overrun. Fix it by splitting out the dirty bitmap into a
separate bitmap per slot.

Fixes: 609e6202 ("KVM: selftests: Support multiple slots in dirty_log_perf_test")
Signed-off-by: David Matlack <dmatlack@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20210917173657.44011-4-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>