KVM: arm64/sve: Allow userspace to enable SVE for vcpus

Now that all the pieces are in place, this patch offers a new flag
KVM_ARM_VCPU_SVE that userspace can pass to KVM_ARM_VCPU_INIT to
turn on SVE for the guest, on a per-vcpu basis.

As part of this, support for initialisation and reset of the SVE
vector length set and registers is added in the appropriate places.
Allocation SVE registers is deferred until kvm_arm_vcpu_finalize(),
by which time the size of the registers is known.

Setting the vector lengths supported by the vcpu is considered
configuration of the emulated hardware rather than runtime
configuration, so no support is offered for changing the vector
lengths of an existing vcpu across reset.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>