tools: Set host as default target

BUG=None
TEST= ran test_runner on windows. Verfied in presubmit

Change-Id: Id4cd9002ea01be5cf48b122543d3458be3a634f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3724616
Commit-Queue: Vikram Auradkar <auradkar@google.com>
Reviewed-by: Richard Zhang <rizhang@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dennis Kempin <denniskempin@google.com>

tools/impl/test_runner.py

@@ -371,6 +371,7 @@ def main():
     )
     parser.add_argument(
         "--target",
+        default="host",
         help="Execute tests on the selected target. See ./tools/set_test_target",
     )
     parser.add_argument(

tools/impl/test_target.py

@@ -114,7 +114,7 @@ class TestTarget(object):
     """A test target can be the local host, a VM or a remote devica via SSH."""
 
     target_str: str
-    is_host: bool = False
+    is_host: bool = True
     vm: Optional[testvm.Arch] = None
     ssh: Optional[Ssh] = None
     __arch: Optional[Arch] = None
@@ -131,10 +131,12 @@ class TestTarget(object):
             arch: testvm.Arch = parts[1]  # type: ignore
             self.vm = arch
             self.ssh = Ssh("localhost", testvm.ssh_cmd_args(arch))
+            self.is_host = False
         elif len(parts) == 2 and parts[0] == "ssh":
             self.ssh = Ssh(parts[1])
+            self.is_host = False
         elif len(parts) == 1 and parts[0] == "host":
-            self.is_host = True
+            pass
         else:
             raise Exception(f"Invalid target {target_str}")
         if build_arch: