KVM: arm/arm64: Rename soft timer to bg_timer

As we are about to introduce a separate hrtimer for the physical timer,
call this timer bg_timer, because we refer to this timer as the
background timer in the code and comments elsewhere.

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