KVM: arm64/sve: Add pseudo-register for the guest's vector lengths

This patch adds a new pseudo-register KVM_REG_ARM64_SVE_VLS to
allow userspace to set and query the set of vector lengths visible
to the guest, along with corresponding storage in struct
kvm_vcpu_arch.

In the future, multiple register slices per SVE register may be
visible through the ioctl interface.  Once the set of slices has
been determined we would not be able to allow the vector length set
to be changed any more, in order to avoid userspace seeing
inconsistent sets of registers.  For this reason, this patch adds
support to track vcpu finalization explicitly, and enforce proper
sequencing of ioctls.

The new pseudo-register is not exposed yet.  Subsequent patches
will allow SVE to be turned on for guest vcpus, making it visible.

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