st/oxr: fix CTS warning about unsupported view configs
Fixes the following CTS warning messages:
"On receiving a 'valid' but not supported enum value 1000037000,
the runtime returned as XR_ERROR_VALIDATION_FAILURE instead of
XR_ERROR_VIEW_CONFIGURATION_TYPE_UNSUPPORTED, which may make it
harder for apps to reason about the error.
viewType := XR_VIEW_CONFIGURATION_TYPE_PRIMARY_STEREO_WITH_FOVEATED_INSET"
Part-of: <https://gitlab.freedesktop.org/monado/monado/-/merge_requests/2402>
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