install libvirt as a dependency of nova

nova compute depends on libvirt, but it was not being installed
in the case of LIBVIRT_TYPE=lxc.

Change-Id: Iedae29e476ad529daa7c7b1be39a58a1c86c3b7c
diff --git a/stack.sh b/stack.sh
index 5e3432d..c7a6102 100755
--- a/stack.sh
+++ b/stack.sh
@@ -699,6 +699,7 @@
 
     # Virtualization Configuration
     # ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+    apt_get install libvirt-bin
 
     # attempt to load modules: network block device - used to manage qcow images
     sudo modprobe nbd || true
@@ -707,7 +708,6 @@
     # kvm, we drop back to the slower emulation mode (qemu).  Note: many systems
     # come with hardware virtualization disabled in BIOS.
     if [[ "$LIBVIRT_TYPE" == "kvm" ]]; then
-        apt_get install libvirt-bin
         sudo modprobe kvm || true
         if [ ! -e /dev/kvm ]; then
             echo "WARNING: Switching to QEMU"