blockdev-test: add test case for drive_add duplicate IDs

The following should work:

  (qemu) drive_add if=none,id=drive0
  (qemu) drive_del drive0
  (qemu) drive_add if=none,id=drive0

Previous versions of QEMU produced a duplicate ID error because
drive_add leaked the options.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
