KVM: selftests: Return created vcpu from vm_vcpu_add_default()

Return the created 'struct kvm_vcpu' object from vm_vcpu_add_default(),
which cleans up a few tests and will eventually allow removing vcpu_get()
entirely.

Opportunistically rename @vcpuid to @vcpu_id to follow preferred kernel
style.

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