spec: don't use chkconfig --list

spec: don't use chkconfig --list

https://bugzilla.redhat.com/show_bug.cgi?id=694403 reports that
the specfile is incorrectly checking for a running libvirt-guests
service.  For example,

$ LC_ALL=es_ES chkconfig --list libvirt-guests
libvirt-guests 	  0:desactivado		 1:desactivado	   2:desactivado     3:activo	 4:activo  5:activo	    6:desactivado

will fail to find 5:on, even though it is active.  But chkconfig
already has a mode where you can silently use the exit status to
check for an active service.

* libvirt.spec.in (%post): Use simpler chkconfig options, to avoid
issues with localization.