Initialise the connection ID when a domain is introduced. This (re)enables

Initialise the connection ID when a domain is introduced.  This (re)enables
the permission checking in xenstored.

Default the store permissions to read/write nobody (apart from the privileged
domain).  Create a /local node with these permissions, ready for inheriting by
children.  In Xend, create a /vm node with these permissions too, for the same
reason, and set the permissions on /local/domain/<domid> and each device
backend path to allow the guest domain to access these paths appropriately.

Added xstransact.{set_permissions,SetPermissions,mkdir,Mkdir,complete} as
support facilities.

This closes bug #290.

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