Detect when Debian is used to launch libvirtd

this patch checks if os_VENDOR is ubuntu to launch
libvirt-bin service. Previously, is_ubuntu() was used, but
this function only detects if a deb packaging is used, so
there were no distinction between Ubuntu and Debian.

Change-Id: I222b71962f49896063910ff2a25e4f57be4bf819
Closes-Bug: 1361260

Author: matrohon

lib/nova_plugins/functions-libvirt

@@ -57,7 +57,7 @@ cgroup_device_acl = [
 EOF
     fi
 
-    if is_ubuntu; then
+    if [ "$os_VENDOR" = "Ubuntu" ]; then
         LIBVIRT_DAEMON=libvirt-bin
     else
         LIBVIRT_DAEMON=libvirtd