core: Event should log disk alias instead UNKNOWN.

When removing a disk, we should indicate the disk name instead <UNKNOWN>
string.
The <UNKNOWN> is presented since engine tries to get the disk
alias after the disk was already deleted.
The fix is to initialize the disk parameter before the deletion.

Change-Id: Id111a067819cd881ee6fd22b588562028087fee8
Signed-off-by: Maor Lipchuk <mlipchuk@redhat.com>