core: Add disk device after disk is added

Currently, AddDiskCommand adds the device to the DB,
before the disk is there, this cause the boot order not to take the new
disk into consideration.

In this patch the addition of the device is moved after the disk is
created by the disk is in the db.

Change-Id: I2f2fc8e6771ed2b352f2bde7b4e313481df76b90
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1131018
Signed-off-by: Omer Frenkel <ofrenkel@redhat.com>