Usage message now goes to stdout to enable easier use with grep. New

Usage message now goes to stdout to enable easier use with grep.  New
configuration variable $QEMU_NBD (required on Ubuntu since their KVM
package renames the binary to kvm-nbd).  vircmd no longer needs
config.default - will now accept a cluster name and work out the node
names as before, or will accept a glob-style wildcard to specify
nodes.

Signed-off-by: Martin Schwenke <martin@meltin.net>