libxl: allow the storage backend to choose between qdisk/blkback

We remove the 'use_qdisk' global option because we want the storage
manager to decide what the backend should look like. Note that with
libxl we have 2 choices:

- blkback
- qdisk

There is no tapdisk3 support in libxl.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>