qemu: process: Fix alias for disk-tray-moved event

qemu: process: Fix alias for disk-tray-moved event

Currently we'd report the alias of the drive which is backing the cdrom
rather than the device itself:

 $ virsh event ds tray-change --loop
 event 'tray-change' for domain ds disk drive-ide0-0-1: opened
 event 'tray-change' for domain ds disk drive-ide0-0-1: closed

Report the disk device alias as we document in the API docs:

https://libvirt.org/html/libvirt-libvirt-domain.html#virConnectDomainEventTrayChangeCallback

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>