scsi: fix drive hotplug.

scsi: fix drive hotplug.

This patch fills the DriveInfo->unit after hotplugging a scsi disk.
It makes a difference when auto-assigning a scsi id, where unit was
left filled with '-1' instead of the actual scsi id.

With this patch applied the the drive naming logic in drive_init() works
as good as it did in previous releases.  Which means it works fine with
a single scsi bus.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 11f4d7f483d9ca3a8979abd605cf1468b3e96aa2)