Use a shorter name for the top-level vimage container holding all

Use a shorter name for the top-level vimage container holding all
virtual nodes in an experiment, so that a string in form of
ifname@eid.nodeid could fit into 15 characters or less for all node
ids smaller than n1000.  The 15 chatacter string length limitation
is derived from the IF_NAMESIZE constant which applies when specifying
interface names.  Longer eids were previously causing trouble when
attempting to open a bpf tap in child nodes.

This change fixes bugzilla bug #2:
http://imunes.tel.fer.hr/bugzilla/show_bug.cgi?id=2

Bug found by:	miljenko at tel.fer.hr
Submitted by:
Reviewed by:
Approved by:
Obtained from: