qcow2 disks have a maximum size of 2^63 - 513

This is due to a command-line parsing artifact in qemu-img:

http://rwmj.wordpress.com/2011/10/03/maximum-qcow2-disk-size/

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Reject SR.attach with unrecognised image formats

If no default image format is provided (format=XXX) in the device_config
then we will still default to vhd.

If an unrecognised default image format is provided then we raise an
Unimplemented error.

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