iotests: Restrict nbd Python tests to nbd

iotests: Restrict nbd Python tests to nbd

We have two Python unittest-style tests that test NBD.  As such, they
should specify supported_protocols=['nbd'] so they are skipped when the
user wants to test some other protocol.

Furthermore, we should restrict their choice of formats to 'raw'.  The
idea of a protocol/format combination is to use some format over some
protocol; but we always use the raw format over NBD.  It does not really
matter what the NBD server uses on its end, and it is not a useful test
of the respective format driver anyway.

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