core: validation error on edit instance type

The default instance types has been created with min_allocated_mem set to 0
which was incorrect making inpossible to edit them (due to validation error).

Fixed by setting the same min_allocated_mem as mem_size_mb for all instance
types which has the min_allocated_mem set to 0.

Change-Id: If3692e187f04c5a8e7f8c812850cc82af7bc43ab
Signed-off-by: Tomas Jelinek <tjelinek@redhat.com>