KVM: arm64: Free sve_state via arm specific hook

Add an arm specific hook to free the arm64-only sve_state.  Doing so
eliminates the last functional code from kvm_arch_vcpu_uninit() across
all architectures and paves the way for removing kvm_arch_vcpu_init()
and kvm_arch_vcpu_uninit() entirely.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>