Export more information via LUQueryInstances/RAPI

Currently, the custom instance parameters (hv, be, nicp) are only
queryable via LUQueryInstanceData. LUQueryInstance returns only the
filled parameters, thus its users (especially RAPI) have no way to know
if a parameter is custom or the default value.

This patch adds three new parameters: custom_hvparams, custom_beparams,
custom_nicparams, that are also exported in RAPI.

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