Debian: use libvirt-bin when needed

libvirtd is the new name of the init script in Debian testing.
libvirt-bin is still in use on Debian Wheezy.

Since I222b71962f49896063910ff2a25e4f57be4bf819, libvirtd is the
default for Debian, this break the compatibility with Debian Wheezy.

With this patch, we use libvirt-bin only if there is no
/etc/init.d/libvirtd init script.

Change-Id: I13694fef93d36c2e128e15e7dbfaec9230335585
diff --git a/lib/nova_plugins/functions-libvirt b/lib/nova_plugins/functions-libvirt
index f722836..6b9db48 100644
--- a/lib/nova_plugins/functions-libvirt
+++ b/lib/nova_plugins/functions-libvirt
@@ -57,7 +57,9 @@
 EOF
     fi
 
-    if [ "$os_VENDOR" = "Ubuntu" ]; then
+    # Since the release of Debian Wheezy the libvirt init script is libvirtd
+    # and not libvirtd-bin anymore.
+    if is_ubuntu && [ ! -f /etc/init.d/libvirtd ]; then
         LIBVIRT_DAEMON=libvirt-bin
     else
         LIBVIRT_DAEMON=libvirtd