apic: Open-code timer save/restore

To enable migration between accelerated and non-accelerated APIC models,
we will need to handle the timer saving and restoring specially and can
no longer rely on the automatics of VMSTATE_TIMER. Specifically,
accelerated model will not start any QEMUTimer.

This patch therefore factors out the generic bits into apic_next_timer
and use a post-load callback to implemented model-specific logic.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>