KVM: arm64: Let vcpu_sve_pffr() handle HYP VAs

The vcpu_sve_pffr() returns a pointer, which can be an interesting
thing to do on nVHE. Wrap the pointer with kern_hyp_va(), and
take this opportunity to remove the unnecessary casts (sve_state
being a void *).

Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>