KVM: LAPIC: Loosen filter for adaptive tuning of lapic_timer_advance_ns

5000 guest cycles delta is easy to encounter on desktop, per-vCPU
lapic_timer_advance_ns always keeps at 1000ns initial value, let's
loosen the filter a bit to let adaptive tuning make progress.

Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>