KVM: arm: Handle VCPU device attributes in guest.c

As we are about to support VCPU attributes to set the timer IRQ numbers
in guest.c, move the static inlines for the VCPU attributes handlers
from the header file to guest.c.

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