update xterm_prefix for latest psutil version

[nealseegmiller]

xterm_prefix in swri_prefix_tools calls .get_children() on a psutil Process object. The name of this function has been changed to .children() in the latest version:
http://grodola.blogspot.com/2014/01/psutil-20-porting.html
Maybe other changes are required, because I made this change and the function still fails to find any children. The function terminates at:
if grandchild is None:
print "Found no running process for node. Quitting."
sys.exit(1)

FYI, I'm using swri_prefix_tools in combination with swri_nodelet, in case that's relevant.