ARM: KVM: Move VFP registers to a CPU context structure

In order to turn the WS code into something that looks a bit
more like the arm64 version, move the VFP registers into a
CPU context container for both the host and the guest.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>