Catch Storage_interface.Backend_error and pretty-print.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
vdi-create takes optional argument --format=...

This argument requests the storage backend to use a specific
storage format, typically 'vhd' or 'raw'.

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

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Allow sr-attach configuration parameters to be read from files.

When an SR expects configuration key=value, we can now make a file
called 'foo' with contents 'value' and write:

  key-filename=foo

This is expected to be particularly useful when an SR requires
an XML configuration string (consider libvirt)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update to 0.9.1

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