KVM: selftests: Convert memslot_perf_test away from VCPU_ID

KVM: selftests: Convert memslot_perf_test away from VCPU_ID

Convert memslot_perf_test to use __vm_create_with_one_vcpu() and pass
around a 'struct kvm_vcpu' object instead of using a global VCPU_ID.
This is the first of many, many steps towards eliminating VCPU_ID from
all KVM selftests, and towards eventually purging the VM+vcpu_id mess.

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