Fix usleep definition -- we would be sleeping 1000 times too long if the usleep

Fix usleep definition -- we would be sleeping 1000 times too long if the usleep
command were not available elsewhere on the machine (on my Debian boxes, for
example).

Sleep for a configurable amount of time between xm create calls (by default,
5 seconds).  This spaces out the booting of the new domains, meaning that they
should not thrash the disk so much.

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