Adding output to 'virsh --version=long' makes it easier to
tell if a distro built with particular libraries (it doesn't
tell you what a remote libvirtd is built with, but is still
better than nothing).  But we forgot to mention gluster.

* tools/virsh.c (vshShowVersion): Add gluster witness.
Signed-off-by: NEric Blake <eblake@redhat.com>