In oneVPL, a valid mfxLoader handle is needed when creating mfx session
for decoding, encoding and processing[1], so add loader field to
AVQSVDeviceContext. User should fill this field before calling
av_hwdevice_ctx_init() if using oneVPL

This is in preparation for oneVPL support

[1]https://spec.oneapi.io/versions/latest/elements/oneVPL/source/programming_guide/VPL_prg_session.html#onevpl-dispatcher