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