xenapi: removes sleep after final upload retry

This change removes the sleep that happens after the final retry of
uploading an image.

This change also ensures the unit tests do not take too long by stubbing
out time.sleep.

Closes bug 1239707
Change-Id: I16b6639e3231d41fa476218f406fa5b3abdfaf71
Merge "xenapi: removes sleep after final upload retry"