Remove unsupported systemd.socket settings

The libvirtd.socket unit file contains SocketUser and SocketGroup
settings that were introduced in systemd version 214.  Remove the
setting to prevent unnecessary and confusing journal entries such
as

  Unknown lvalue 'SocketGroup' in section 'Socket'

See bsc#933043

Index: libvirt-1.2.21/daemon/libvirtd.socket.in
===================================================================
--- libvirt-1.2.21.orig/daemon/libvirtd.socket.in
+++ libvirt-1.2.21/daemon/libvirtd.socket.in
@@ -2,10 +2,8 @@
 ListenStream=@runstatedir@/libvirt/libvirt-sock
 ListenStream=@runstatedir@/libvirt/libvirt-sock-ro
 
-; The following settings must match libvirtd.conf file in order to
-; work as expected because libvirtd can't change them later.
+; The following setting must match libvirtd.conf file in order to
+; work as expected because libvirtd can't change it later.
 ; SocketMode=0777 is safe only if authentication on the socket is set
 ; up.  For further information, please see the libvirtd.conf file.
 SocketMode=0777
-SocketUser=root
-SocketGroup=root