virsh: remove xen reference in header comment

With libvirt and virsh now being used for much more than Xen, this
patch removes the outdated reference to Xen in the file header.

tools/virsh.c

 /*
- * virsh.c: a Xen shell used to exercise the libvirt API
+ * virsh.c: a shell to exercise the libvirt API
  *
  * Copyright (C) 2005, 2007-2010 Red Hat, Inc.
  *