trace: make `trace extract` silent

The command runs GDB which is very chatty. This will spoil our clean
output of 'make check' once the tracing test is added. Let's hide GDB
output unless it fails or the tracefile is not created.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>

scripts/trace.py

@@ -213,8 +213,16 @@ def extract(args):
     else:
         elf_path = 'build/release/loader.elf'
 
-    if (os.path.isfile(elf_path)):
-       sys.exit(os.system("gdb %s -batch -ex conn -ex \"osv trace save %s\"" % (elf_path, args.tracefile)))
+    if os.path.isfile(elf_path):
+        if os.path.exists(args.tracefile):
+            os.remove(args.tracefile)
+            assert(not os.path.exists(args.tracefile))
+        proc = subprocess.Popen(['gdb', elf_path, '-batch', '-ex', 'conn', '-ex', 'osv trace save ' + args.tracefile],
+            stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+        _stdout, _ = proc.communicate()
+        if proc.returncode or not os.path.exists(args.tracefile):
+            print(_stdout)
+            sys.exit(1)
     else:
         print("error: %s not found" % (elf_path))
         sys.exit(1)