virttest: Update khugepaged test timeout

As the khugepaged process will quit slowly in some OS, enlarge the
timeout for khugepaged test from 5s to 50s. And also use the method that
check process status in a while loop which will break when the khugepaged
process status is changed as expect instead of time.sleep(timeout).

Signed-off-by: Yiqiao Pu <ypu@redhat.com>