core: GetAllDisksByVmIdQuery - proper list copying

Create the result list by passing the list from the DAO to the
constructor instead of iterating and calling add(disk).
This way we ensure the list is exactly the right size, and no wasted
memory or redundant regrows occur.

Change-Id: I3402084258aebf3cb9dcfa1d4c494281e95aeeb8
Signed-off-by: Allon Mureinik <amureini@redhat.com>