The host part of a block device can be deleted with in progress
block migration.

To fix this, add a reference count to DriveInfo, freeing resources
on last reference.
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>