KVM: selftests: Return the created vCPU from vm_vcpu_add()

Return the created vCPU from vm_vcpu_add() so that callers don't need to
manually retrieve the vCPU that was just added.  Opportunistically drop
the "heavy" function comment, it adds a lot of lines of "code" but not
much value, e.g. it's pretty obvious that @vm is a virtual machine...

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