iotests: Restrict file Python tests to file

iotests: Restrict file Python tests to file

Most of our Python unittest-style tests only support the file protocol.
You can run them with any other protocol, but the test will simply
ignore your choice and use file anyway.

We should let them signal that they require the file protocol so they
are skipped when you want to test some other protocol.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>