Instance transfer: add argument for the 'component'

Currently, transfer data is done mainly with just the instance name,
but when we have instances with multiple disks this is not enough to
distinguish between the different transfers being done for the
instance.

Some parts of the code do have knowledge of the part being transferred
(i.e. DiskTransfer.name), but if I understood correctly not all, so I
decided to add a new argument to the respective disk import/disk
export classes.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>