[CORE] vmm_timer: Further improve sanity check in vmm_timer_timestamp()

It can happen that vmm_timer_timestamp() is called after per-cpu
timer control structure is zeroed-out but before per-cpu time counter
is initialized hence we need to ensure that per-cpu timer counter is
initialized before it is used by vmm_timer_timestamp().

Signed-off-by: Anup Patel <anup@brainfault.org>