meson/proxy: add a devenv option to start ad-hoc server
The new flow if not installing server:
- meson out -Dvenus
- ninja -C out
- meson devenv -C out
- ./vtest/virgl_test_server --venus
- exit
Signed-off-by: Yiwei Zhang zzyiwei@chromium.org
Closes: #346 (closed)