target-arm: Convert TCG to using (index,value) list for cp migration

Convert the TCG ARM target to using an (index,value) list for migrating
coprocessors. The primary benefit of the (index,value) list is for
passing state between KVM and QEMU, but it works for TCG-to-TCG
migration as well and is a useful self-contained first step.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>