--gpu_mode="drm_virgl" seems to be broken (again) ?

This issues seems once be brought up by @nylon7 in #85 .

I am encountering it yet again with the latest code base without the rutabaga device.

After switching the gpu mode to `drm_virgl`, CVD fails to show the display and logcat suggests setting `ro.hardware.egl`.
I then comply by setting it to `mesa`, adding libEGL*_mesa* to the product settings.
However, the logcat still says :
```
01-01 00:02:19.730 485 529 D libEGL : Failed to load drivers from property ro.hardware.egl with value mesa
01-01 00:02:19.733 485 529 F libEGL : couldn't find an OpenGL ES implementation, make sure one of persist.graphics.egl, ro.hardware.egl and ro.board.platform is set
```

My command line with virgl piping - -
```
DISPLAY=:33 GALLIUM_DRIVER=virpipe launch_cvd -cpus=4 --memory_mb=8192 --resume=false --gpu_mode="drm_virgl" --console=true
```

The X server is plain Xvfb. I doubly checked the virpipe setting is working with ordinary X programs without problems.
And the cvd launcher log doesn't complains anything.

The logs are attached.
[launcher.log](https://github.com/google/android-riscv64/files/14987870/launcher.log)
[logcat.log](https://github.com/google/android-riscv64/files/14987781/logcat.log)

====

I know that the drm_virgl GPU mode is removed from README in favor of the newly merged QEMU rutabaga device and GFX stream.
However, it could be quite difficult for people working with offshoots of QEMU code base.
So the need for drm_virgl is still strong.