Fix typo

Signed-off-by: John Else <john.else@citrix.com>
VDI.copy: add a 'driver_params' to specify implementation-specific extras

This is currently plumbed-through and ignored.

We will use this to allow vhd-specific options to be sent, including
a request to copy only the deltas between two VDIs (eg a base disk
and a snapshot) in order to make snapshot-export quick.

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