ARM KVM: save and load VFP registers from kernel

Add support for saving and restoring VFP register state from the
kernel. This includes a check that the KVM-created CPU has full
VFP support (as the TCG Cortex-A15 model always does), since for
the moment ARM QEMU doesn't have any way to tweak optional features
on created CPUs.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
