core: introudcing domain ovf handling commands

*ProccessOvfUpdateForStorageDomainCommand -
Used for perform ovf update for the valid ovf stores of a storage
domain, creates more ovf stores if needed (there are less than defined
        in the config file)

*CreateOvfVolumeForStorageDomainCommand -
Command used for creating ovf store disk for a given domain.

Change-Id: Ifea111e6732ec0b6953d54ef03c29eb0dff6f837
Signed-off-by: Liron Aravot <laravot@redhat.com>