Log system() failures in ovirtfunctions

Don't just silently return False. Log stderr and the output if
the command fails, so we have some idea that it failed and why.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1158044
Change-Id: Icd1a8d425944c19e46e6189ae0983346d0154805
Signed-off-by: Ryan Barry <rbarry@redhat.com>