add a --quiet flag

test-gdb and test-userland produce beautiful output by default

create def get_common_args to help forward common args to child calls...
it is ugly, but I'm lazy for a perfect solution now

build-qemu

@@ -9,7 +9,7 @@ class Main(common.BuildCliFunction):
     def __init__(self):
         super().__init__()
         self.add_argument(
-            '--userland',
+            '--user-mode',
             default=False,
             help='Build QEMU user mode instead of system.',
         )
@@ -27,7 +27,7 @@ class Main(common.BuildCliFunction):
             verbose = ['V=1']
         else:
             verbose = []
-        if self.env['userland']:
+        if self.env['user_mode']:
             target_list = '{}-linux-user'.format(self.env['arch'])
         else:
             target_list = '{}-softmmu'.format(self.env['arch'])