ovirt-init-functions: executes on-boot hooks if exists

Currently, we try to run on-boot hooks even if there are
no hooks files in the on-boot dir.
This patchs adds a validation to look for on-boot hooks
if the dir is not empty.

Change-Id: Iefa5ca7614c1a7687dd0b51a3a51ec7015853152
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>