backend: GetoVirtISOsQuery fix null for nodeOS

This patch adds validation for nodeOS to avoid
the null error when we cannot collect hostOS.

Change-Id: If8ad4f88a079c7f25b544320e10f1d2b5c4190b2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1076221
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>