core: cleanup in ovf readers where hardware is read

This patch reduces the duplicated code in template's OVF reader and VM's
OVF reader which is related to the reads of the hardware section.

The common code is now in OvfReader, and OvfTemplateReader & OvfVmReader
overrides it when needed.

Change-Id: Ia44e11f4c6eca2dc47ed2ee24bd855d7888fe015
Signed-off-by: Arik Hadas <ahadas@redhat.com>