New pylint warns:
  boot-qemu.py:401:12: W0719: Raising too general exception: Exception (broad-exception-raised)
  boot-qemu.py:431:16: W0719: Raising too general exception: Exception (broad-exception-raised)
The type of exception really does not matter, as we do not expect this script
to be called, so the exception will never be caught. However, it is easy to
silence this by switching to a more specific exception. FileNotFoundError is
descriptive so use this exception to silence the warning.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>

New pylint warns:
  boot-qemu.py:413:8: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)
  boot-qemu.py:418:8: R1732: Consider using 'with' for resource-allocating operations (consider-using-with)
We do not expect these calls to fail but the refactoring is simple to silence
the warning.
Signed-off-by: Nathan Chancellor <nathan@kernel.org>