1] You should upgrade the cairo-1.12.14-x86_64-1_slack14.1 package to cairo-1.12.16-x86_64-1_slack14.1. The original Cairo package that shipped with Slackware64 14.1 will cause a display issue in Virt-manager. To do this:

su - # Or login as root.
slackpkg update # You should have a mirror enabled before you do this.
slackpkg upgrade-all

2] For the necessary software, you may use the binaries I compiled. If you would like to take this path, do the following immediately after step 1:

cd ~
wget http://www.malaya-digital.org/slackware64-14.1-kvm.tar.gz
mkdir kvm
cd kvm
tar xzpf ../slackware64-14.1-kvm.tar.gz
installpkg *.tgz

- OR -

If you don't want to use my binaries, you may compile (and then install immediately after compiling) the following software from slackbuilds.org in the order shown:

celt051
device-tree-compiler
libcacard
spice-protocol
pyparsing
spice
usbredir
vala
yajl
qemu
gnome-python2-gconf
gtk-vnc
ipaddr-py
libvirt
libvirt-glib
libvirt-python
pygobject3
spice-gtk
tunctl
urlgrabber
virt-manager
vte3

3] Append this in your /etc/rc.d/rc.local:

/etc/rc.d/rc.libvirt start

4] Make sure you have a /etc/rc.d/rc.local_shutdown that at least have the following:

#!/bin/bash
/etc/rc.d/rc.libvirt stop

5] Reboot Slackware.