core: avoid adding ovf store when register disk fails

Trying to add an ovf store when the disk registration failed it will
result in a low level sql error:

 ERROR: insert or update on table "storage_domains_ovf_info" violates
   foreign key constraint "storage_domains_ovf_info_ovf_disk_id_fkey"

Change-Id: I947a04e14305cd96afa8c455391f2084856d1fc5
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>