I've created a wrapper for Samba that seems to do the trick, with a big
caveat. It doesn't error out when Samba is not actually available.

I'd prefer adding Samba directely, but if that's not an option it's
better than nothing. I'll send it in later this week after cleaning it
up a bit.

On Wed, 27 Dec 2017 14:23:31 +0100
Rutger Helling <rhelling@mykolab.com> wrote:

> Sorry, I hadn't thought of that! I've reverted it for now.
> 
> Would another option be to inherit to a new package? Something like
> qemu-full, where we could more easily turn on all the bells and
> whistles that Qemu has?
> 
> On Wed, 27 Dec 2017 13:04:09 +0100
> ludo@gnu.org (Ludovic Courtès) wrote:
> 
> > Hello,
> > 
> > rhelling@mykolab.com (Rutger Helling) skribis:
> >   
> > > commit 84276503059d46c0a42240f8770f59090614d863
> > > Author: Rutger Helling <rhelling@mykolab.com>
> > > Date:   Wed Dec 27 10:09:26 2017 +0100
> > >
> > >     gnu: qemu: Enable Samba support.
> > >     
> > >     * gnu/packages/virtualization.scm (qemu)[arguments]: Change
> > > #:configure-flags to a list, add --smbd flag.
> > >     [inputs]: Add samba.    
> > 
> > [...]
> >   
> > > +       #:configure-flags (list "--enable-usb-redir"
> > > "--enable-opengl"
> > > +                               (string-append "--smbd="
> > > +                                              (assoc-ref
> > > %build-inputs "samba")
> > > +                                              "/sbin/smbd"))    
> > 
> > For the record, we removed the dependency on Samba long ago (commit
> > b344c505f4dff2ecbe981f0a0a3c3d67b222dcca), largely because Samba is
> > a huge dependency:
> > 
> > --8<---------------cut here---------------start------------->8---
> > $ guix size qemu | tail -1
> > total: 817.4 MiB
> > $ guix size qemu samba | tail -1
> > total: 1176.0 MiB
> > --8<---------------cut here---------------end--------------->8---
> > 
> > As it stands I’d still be in favor of not having the default ‘qemu’
> > depend on Samba.
> > 
> > Perhaps one solution would be to change the ‘exec’ call that invokes
> > ‘smbd’ to ‘execlp’ (or similar) so that ‘smbd’ is search for in
> > $PATH. That way, if the user installs Samba, then support is
> > available, otherwise it’s missing.
> > 
> > WDYT?
> > 
> > Thanks,
> > Ludo’.  
>