fence_ibmz: fix handling of API response when LPAR is not found

According to its documentation the HMC API should return an
empty array when an LPAR is not found but instead for CPCs in classic
mode it returns a 404 response. This patch changes the
handling of the response to handle the situation accordingly.

Signed-off-by: Paulo de Rezende Pinatti <ppinatti@linux.ibm.com>