KVM: arm64: Introduce vcpu_sve_vq() helper

The KVM code contains a number of "sve_vq_from_vl(vcpu->arch.sve_max_vl)"
instances, and we are about to add more.

Introduce vcpu_sve_vq() as a shorthand for this expression.

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