[CORE] vmm_timer: Make timer event operations SMP safe

Till no we only have per-CPU active event list which as protected
by spinlock for SMP safe access but on SMP host the timer event
start/stop operations also need to be SMP safe.

This patch adds a spinlock to timer event for making event start
and stop operations SMP safe.

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