KVM: arm/arm64: Set a background timer to the earliest timer expiration

When scheduling a background timer, consider both of the virtual and
physical timer and pick the earliest expiration time.

Signed-off-by: Jintack Lim <jintack@cs.columbia.edu>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>