conf: Restore setting default bus for input devices

conf: Restore setting default bus for input devices

Prior to v9.3.0-rc1~30 we used to set default bus for <input/>
devices, during XML parsing. In the commit this code was moved to
a post parse callback. But somehow the line that sets the bus in
one specific case disappeared. Bring it back.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/577
Fixes: c4bc4d3b82fbe22e03c986ca896090f481df5c10
Signed-off-by: Jonathan Wright <jonathan@almalinux.org>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>