daemon: Allow overriding NOFILES ulimit for the daemon as well

One of my latest patches (d8db0f9690) created support for setting
the limit for the maximum of opened files by qemu user. However,
since libvirtd keeps one FD opened per domain (well, for qemu at least)
it will likely hit this limit on huge scenarios.

daemon/libvirtd.upstart

@@ -31,6 +31,11 @@ script
         ulimit -c "$DAEMON_COREFILE_LIMIT"
     fi
 
+    # LIBVIRTD_NOFILES_LIMIT from /etc/sysconfig/libvirtd is not handled
+    # automatically
+    if [ -n "$LIBVIRTD_NOFILES_LIMIT" ]; then
+        ulimit -n "$LIBVIRTD_NOFILES_LIMIT"
+    fi
     mkdir -p /var/cache/libvirt
     rm -rf /var/cache/libvirt/*