KVM: selftests: Convert memop away from VCPU_ID

KVM: selftests: Convert memop away from VCPU_ID

Pass around a 'struct kvm_vcpu' object instead of a vCPU ID in s390's
memop test.  Pass NULL for the vCPU instead of a magic '-1' ID to
indicate that an ioctl/test should be done at VM scope.

Rename "struct test_vcpu vcpu" to "struct test_info info" in order to
avoid naming collisions (this is the bulk of the diff :-( ).

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