blockdev: Separate ID generation from DriveInfo creation

blockdev-add shouldn't automatically generate IDs, but will keep most of
the DriveInfo creation code.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>