guestfs: With libguestfs >= v1.41.1 decode returned bytes to string

libguestfs >= v1.41.1 and commit 0ee02e0117527 changed the return type
of read_file from string to bytes.

libguestfs/libguestfs@0ee02e0

As we don't check the version of libguestfs installed this change
handles both the original behaviour where a string is returned and the
newer behaviour by decoding any returned bytes to a string.

Closes-Bug: #1882421
Change-Id: I1c12b2903c1e5bf3a88394493456ad33233f3cd8

Author: lyarwood

nova/tests/unit/virt/disk/vfs/fakeguestfs.py

@@ -109,7 +109,7 @@ def read_file(self, path):
                 "mode": 0o700
                 }
 
-        return self.files[path]["content"]
+        return bytes(self.files[path]["content"].encode())
 
     def write(self, path, content):
         if path not in self.files:

nova/virt/disk/api.py

@@ -615,8 +615,8 @@ def _set_passwd(username, admin_passwd, passwd_data, shadow_data):
 
     :param username: the username
     :param admin_passwd: the admin password
-    :param passwd_data: path to the passwd file
-    :param shadow_data: path to the shadow password file
+    :param passwd_data: Data from the passwd file decoded as a string
+    :param shadow_data: Data from the shadow file decoded as a string
     :returns: nothing
     :raises: exception.NovaException(), IOError()
 

nova/virt/disk/vfs/guestfs.py

@@ -306,7 +306,14 @@ def replace_file(self, path, content):
     def read_file(self, path):
         LOG.debug("Read file path=%s", path)
         path = self._canonicalize_path(path)
-        return self.handle.read_file(path)
+        data = self.handle.read_file(path)
+        # NOTE(lyarwood): libguestfs v1.41.1 (0ee02e0117527) switched the
+        # return type of read_file from string to bytes and as such we need to
+        # handle both here, decoding and returning a string if bytes is
+        # provided. https://bugzilla.redhat.com/show_bug.cgi?id=1661871
+        if isinstance(data, bytes):
+            return data.decode()
+        return data
 
     def has_file(self, path):
         LOG.debug("Has file path=%s", path)