do not expose hardware specific functionality in library interface

[bmegli]

The qp options introduced in ff2b694 is VAAPI specific (codec-private option).

The current implementation of HVE translates to other hardware encoders in a straightforward way (see #5).

By using codec-private options and exposing them in the library interface the translation to other hardware encoders gets difficult, similiar with documentation of the library..

In the long term options such as qp should be probably exposed as vaapi specific or generic options should be used instead and tranlsated to codec specific in implementation.

E.g.

• optional sub-struct in hve_config like vaapi_config, be default NULL or {0}