admin: include: Rename argument dmn to conn in virAdmConnectListServers

admin: include: Rename argument dmn to conn in virAdmConnectListServers

This is just a trivial cosmetic change, throughout all the APIs the argument
is called conn as from 'connection', having 'dmn' at just this single place
might look odd, so better change it now when the interface is still explicitly
disabled and we still have the ability to alter the signatures and public
names before officially enabling it.

Signed-off-by: Erik Skultety <eskultet@redhat.com>