Add a hacky workaround to choose between libxl disk types

We need to use PHY for things which really are block devices but
we can't tell. For now we assume 'xenstore_data=[]' implies block
device.

We need to use QDISK for things which are virtual network block
device things.

Proper fix depends on issue xcp-idl#8

Signed-off-by: David Scott <dave.scott@eu.citrix.com>