This patch adds an --sve command line option to allow the Scalable
Vector Extension to be enabled when creating a guest.

This requires use of the new KVM_ARM_VCPU_FINALIZE ioctl before the
vcpu is runnable, so a new hook kvm_cpu__configure_features() is
added to provide an appropiate place to do this work.

The kernel does not enable SVE by default, and for now kvmtool
adopts the same policy: without --sve, SVE is not enabled for the
guest even if the host supports it.

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