megasas: Use bdrv_drain_all instead of qemu_aio_flush

Calling qemu_aio_flush() directly can hang when combined with I/O
throttling.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Acked-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/megasas.c

@@ -1296,7 +1296,7 @@ static int megasas_dcmd_get_properties(MegasasState *s, MegasasCmd *cmd)
 
 static int megasas_cache_flush(MegasasState *s, MegasasCmd *cmd)
 {
-    qemu_aio_flush();
+    bdrv_drain_all();
     return MFI_STAT_OK;
 }