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

Fixes #8

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Assume files which have a .vhd suffix and no pre-existing metadata, are in vhd format

This mirrors the behaviour for files with an .iso suffix.

Note if a file is created with type=raw but with suffix .vhd, only
the associated metadata makes this safe.

Fixes #9

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.9.7

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