utils.py: Fix joining possible_files in find_first_file()

Fix the following backtrace:

  Traceback (most recent call last):
    File ".../boot-qemu.py", line 870, in <module>
      launch_qemu(config)
    File ".../boot-qemu.py", line 770, in launch_qemu
      virtiofsd = utils.find_first_file(qemu_prefix, virtiofsd_locations)
    File ".../utils.py", line 52, in find_first_file
      f"No files from list ('{', '.join(possible_files)}') could be found within '{relative_root}'!"
  TypeError: sequence item 0: expected str instance, PosixPath found

Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Author: nathanchance

utils.py

@@ -48,8 +48,10 @@ def find_first_file(relative_root, possible_files):
     for possible_file in possible_files:
         if (full_path := relative_root.joinpath(possible_file)).exists():
             return full_path
+
+    files_str = "', '".join([str(elem) for elem in possible_files])
     raise Exception(
-        f"No files from list ('{', '.join(possible_files)}') could be found within '{relative_root}'!"
+        f"No files from list ('{files_str}') could be found within '{relative_root}'!"
     )