Drivers: hv: Explicitly size elements of protocol structures

Use explicitly sized types in data structures defining the host/guest
protocol.

Reported-by: Juan Sanchez-Agrelo <jagrelo@cisco.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>