KVM: selftests: Convert kvm_page_table_test away from reliance on vcpu_id

Reference vCPUs by their 'struct kvm_vcpu' object in kvm_page_table_test
instead of by their ID.  This moves selftests one step closer towards
taking a 'struct kvm_vcpu *' instead of VM+vcpu_id for vCPU helpers.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>