readme: how to configure libvirt after manual installation

When vdsm is installed automatically, libvirt is configured during the
bootstap flow. However, when installing vdsm manually the user must
configure and restart libvirt using vdsm-tool.

Change-Id: I208a752993c0e714a065026155da1f7efa29eeeb
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19761
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>