Make the BlockBackend own the DriveInfo.  Change blockdev_init() to
return the BlockBackend instead of the DriveInfo.
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>