egl emulator: fixes for covegl conformance test

Support for eglChooseConfig with configs==NULL.
Also added small change to query EGL_LARGEST_PBUFFER attrib of surfaces.
Fixed error print in QemuPipeStream::readFully.

Those changes make sure that covegl completes & passes on the emulator with Translator
on linux in the backend.

Change-Id: I892917ec102ae85d56a16a7cb321e8c12e7083f1

tools/emulator/opengl/system/renderControl_enc/renderControl.attrib

@@ -28,6 +28,7 @@ rcChooseConfig
     dir attribs in
 	len attribs attribs_size
 	dir configs out
+    var_flag configs nullAllowed
 	len configs configs_size*sizeof(uint32_t)
 
 rcReadColorBuffer