igvm: Report error on missing parameter area in directive handlers

Parameter areas are how an IGVM file tells QEMU to allocate buffers
for runtime information the guest needs — VP count, memory map,
MADT and so on. Usage directives reference a parameter area by index
to tell QEMU where to write each piece of data. If the index doesn't
match any declared parameter area, the data has nowhere to go and
should be treated as an error.

The directive handlers that look up a parameter area all return 0
(success) when `qigvm_find_param_entry()` can't find it. Therefore,
the load succeeds but the guest never gets the expected parameters.

Note that the IGVM library already validates parameter area indices
when the file is loaded, so this path should only be reachable with
a malformed file that bypassed library validation. This is defensive
programming against that case.

Report the error with error_setg() and return -1 instead.

Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-ID: <20260626-microvm_device_tree-v6-1-9cd13cf057e2@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>