virgl: Document and check expected return values for make_current() callback

The values retuned by Qemu's make_current() callback varies based on
used GL backend. In case of SDL GLX, Qemu returns 0 on success, while for
EGL backends Qemu confusingly returns 1. Document the expected return values
and enforce the new requirement for the callbacks v4, preserving compatibility
for older virglrenderer users implementing v1 callbacks.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Part-of: <!1125>