engine : Clear VM Poweroff/Shutdown reason when RunVmCommand succeeds

Change the way the reason is cleared. When the
RunVmCommand succeeds the reason for the previous
shutdown is cleared.

Change-Id: Ia1fcf297218295b8cae9e5652e32a1351b0ac9e7
Bug-Url: https://bugzilla.redhat.com/1065753
Signed-off-by: Ravi Nori <rnori@redhat.com>