KVM: arm/arm64: Make timer_arm and timer_disarm helpers more generic

We are about to add an additional soft timer to the arch timer state for
a VCPU and would like to be able to reuse the functions to program and
cancel a timer, so we make them slightly more generic and rename to make
it more clear that these functions work on soft timers and not the
hardware resource that this code is managing.

The armed flag on the timer state is only used to assert a condition,
and we don't rely on this assertion in any meaningful way, so we can
simply get rid of this flack and slightly reduce complexity.

Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@linaro.org>