Added xen-script-common.sh, for functions common to all scripts, not just the

Added xen-script-common.sh, for functions common to all scripts, not just the
hotplug ones.  Added evalVariables and findCommand functions to that, which we
use to clarify the handling of variables being passed in as command line
variables.

Make vif-bridge able to find the bridge for itself if only one bridge is in
use.  This means that it is not necessary to specify a bridge in many
configurations.  Allow the bridge to be specified on the command line, meaning
that a default may be provided in the xend-config.sxp if desired.

Added xenstore_read_default to xen-hotplug-common.sh, which reads from the
store but uses a given default if the path in the store is not present.  This
is used by vif-bridge to allow the store details (i.e. those given to xm create)
to override the default value given in the xend-config.sxp.

Remove vif-bridge setting -- the value can be specified on the vif-script
command line if necessary.

Added examples for network-nat/vif-nat.

Added lots of big comments.

Signed-off-by: Ewan Mellor <ewan@xensource.com>