KVM: x86: mmu: remove argument to kvm_init_shadow_mmu and kvm_init_shadow_ept_mmu

The initialization function in mmu.c can always use walk_mmu, which
is known to be vcpu->arch.mmu.  Only init_kvm_nested_mmu is used to
initialize vcpu->arch.nested_mmu.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>