Make nova-compute start after libvirt-guests

On system shutdown libvirt-guests service saves state of every libvirt
instance, including ones managed by OpenStack. But later on startup
nova-compute used to be starting before libvirt-guests and launch all
instances, leaving libvirt-guests no changce to restore state of the
instances from snapshots.

This commit fixes that unfortunate situation by making nova-compute
start after libvirt-guests, taking advantage of the lucky coincidence
that 'n' comes after 'l'.

Change-Id: Ibacc39ede0bb32dabab98ec8eb76e4b6de1cfe3d