[Arm64][libvirt] Fix read bytes from json files

This fix issue that we might get str stream from json files instead of bytes
and make sure we read bytes from json files under
QEMU_FIRMWARE_DESCRIPTOR_PATHS.

Add `rb` mode ([1]) to return contents as bytes objects.
[1] https://docs.python.org/3.8/library/functions.html#open

Closes-Bug: #1921075

Change-Id: I446be58f7f3e972d8e10cc5cbf83ebd42edb34e3

Author: ricolin

nova/tests/unit/virt/libvirt/test_host.py

@@ -1970,4 +1970,4 @@ def test_loaders(self):
             ])
             mock_glob.assert_called_once_with(
                 '/usr/share/qemu/firmware/*.json')
-            mock_open.assert_called_once_with('10_fake.json')
+            mock_open.assert_called_once_with('10_fake.json', 'rb')

nova/virt/libvirt/host.py

@@ -108,7 +108,7 @@ def _get_loaders():
             continue
 
         for spec_path in sorted(glob.glob(f'{path}/*.json')):
-            with open(spec_path) as fh:
+            with open(spec_path, 'rb') as fh:
                 spec = jsonutils.load(fh)
 
             _loaders.append(spec)