ARM: KVM: promote vfp_host pointer to generic host cpu context

We use the vfp_host pointer to store the host VFP context, should
the guest start using VFP itself.

Actually, we can use this pointer in a more generic way to store
CPU speficic data, and arm64 is using it to dump the whole host
state before switching to the guest.

Simply rename the vfp_host field to host_cpu_context, and the
corresponding type to kvm_cpu_context_t. No change in functionnality.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <cdall@cs.columbia.edu>