openstack: plugin hardcodes type 'ext3', so make 'ffs' serve this by default

It's now possible to provide a comma-separated list of queue names
to service. If none are provided then we have builtin defaults:

  org.xen.xcp.storage.ffs
  org.xen.xcp.storage.ext3

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