util: fix success return for virProcessKillPainfullyDelay()

util: fix success return for virProcessKillPainfullyDelay()

virProcessKillPainfullyDelay() currently almost always returns 1 or -1,
even though the documentation indicates that it should return 0 if the
process was terminated gracefully. But the computation of the return
code is faulty and the only case where it currently returns 0 is when it
is called with the pid of a process that does not exist.

Since no callers ever even distinguish between the 0 and 1 response
codes, simply get rid of the distinction and return 0 for both cases.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>