fix audio backend handling

This patch fixes a small bug where the audio backend given with
the -audio <name> option was ignored. The bug was introduced during
a previous integration of upstream audio changes.

The old code always used the same default backend (PulseAudio).

Change-Id: I7bc68df3fd3025f8a8bd1f0fd868872b7eee2017
diff --git a/vl-android.c b/vl-android.c
index acde409..47b802b 100644
--- a/vl-android.c
+++ b/vl-android.c
@@ -5178,10 +5178,13 @@
 
     /* Initialize audio. */
     if (android_op_audio) {
+        char temp[128];
         if ( !audio_check_backend_name( 0, android_op_audio ) ) {
             PANIC("'%s' is not a valid audio output backend. see -help-audio-out",
                     android_op_audio);
         }
+        snprintf(temp, sizeof temp, "QEMU_AUDIO_DRV=%s", android_op_audio);
+        putenv(temp);
     }
 
     /* Initialize OpenGLES emulation */