core: cleanup in ovf readers

- Changed OvfReader#readVmDevice so it will only create VmDevice from
  the given XML node
- Introduced two new methods, one for reading managed device and another
  one for reading unmanaged device
- Extracted the code that adds a given managed VmDevice to separate
  method, addManagedVmDevice (this method will be used in later patches)

Change-Id: I01133cc483252683893d79d403c29aabff6701f5
Signed-off-by: Arik Hadas <ahadas@redhat.com>