staging: unisys: Change data to point to visor_controlvm_parameters_header.

The data field was being defined as a character array and then casted into
a visor_controlvm_parameters_header structure. This patch converts it to
just point to the visor_controlvm_parameters_header structure. The data
following the header is still behind the header_info.

Reported-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>