KVM: arm64: Exclude host_fpsimd_state pointer from kvm_vcpu_arch

As the name of the field indicates, host_fpsimd_state is strictly
a host piece of data, and we reset this pointer on each PID change.

So let's move it where it belongs, and set it at load-time. Although
this is slightly more often, it is a well defined life-cycle which
matches other pieces of data.

Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>