core: Add alias and description for disk meta data

Add the disk alias and the description in the disk meta data.
Those fields should be initialized in the meta data so they can be
preserved and restored for floating disks,
once the Storage Domain was being detached and attached again.

Change-Id: Ie2642ae7016579ead699509426e01ac2010bd374
Bug-Url: https://bugzilla.redhat.com/1047623
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>